Whats your favorite game in the franchise and why?

4, followed by SMAC, followed by 2, and then Freeciv. 3 I have personal disagreements with and Civ 1 is just a bit too antiquated.

I like 4 the most because it manages to strike into that nice rock garden zone of zoning out.

Civilization IV is a game design masterpiece. Its mechanics are so elegantly designed and interact harmoniously with each other. The city maintenance mechanic is brilliant, the best counter to non-stop expansion in the whole series. Settling the right terrain and improving tiles is crucial to master the game. The economy is very deep and many alternatives are viable (cottage, specialist, hybrid and even espionage economy). The civics system adapts to your needs through the game and almost all of them have their chance to shine. The tech tree allows for flexibility and encourages pursuing different branches. Religion spices up diplomacy with the unavoidable formation of religious blocks.

It's also very accessible to newbies, since at the easier difficulty levels it is possible to ignore, automatize or just choose whatever looks cool for intermediate-advanced mechanics (worker management, great people generation, espionage, getting control of the United Nations). However, this is needed to master in order to climb up to higher difficulty levels, and the hardest ones are a real challenge. The addition of random events and other optional features allow both competitive gamers and role-players to enjoy their preferred game style.

Even one its weakest points, the AI, was considered a big improvement at release, and refined later with the Beyond the Sword expansion. It is still the best AI in the Civilization series, since both V and VI failed to teach the AI how to play the game.

And then there's the miriad of mods which will enhance your experience for free.

Most games in the series is excellent but Civilization 3 is my favorite of them all. One of the reasons is that waging war and building a large empire is the most fun in this version. Warfare quickly gets boring and time consuming in Civ 5 and while this works better in Civ 4 it is nowhere as fun or immersive as it is in Civ 3. In what terrain you position your units is an important factor here as well, but you can luckily move large armies without your other¨armies blocking them.
Diplomacy is as fun as it is in Civ 4 and Civ 5 and it is one of the huge improvements over Civ 2.

That said there are some great new things in Civ 4 and especially in Civ 5. Moving away from spamming out cities as the only strategy was a good choice and likewise the greater specialization of the cities and civilizations in later entries. I like that there is a lot of differences between some of the games in the series. I like playing a small civilization in Civ 5 that has only a few cities, but manage to outwit large empires with supreme science or culture but I also like building up a huge empire in Civ 3 from blood, iron and ruthless diplomacy. And actually getting some real resistance from the computer players.
Many of the new things in Civ 6 looks like fun but the art style and interface is very off-putting, so I'm at least waiting until the last expansion pack has been released.

Civ 2 is still alot of fun and has that "classic PC game of the Golden Era" feeling like Sim City 2000 and Caesar 2. But compared to Civilization 3 and later games it feels a bit to shallow and simple. But sometimes a faster, more streamlined game is exactly what you want, hence why you would want to play Heroes of Might and Magic 1 over HoMM2 or HoMM 3. Or (I'm told) Master of Orion over Master of Orion 2.

What surprises me is how much my tastes and viewpoint on this, my favorite video game series of all time, have changed. For most of my Civ-playing life, Civ 2 was always by far my favorite (it's what got me started playing strategy games back in 1996 - I must have put in well over 2000 hours into Civ 2), followed closely by SMAC and Civ 4. However, over the last few weeks I've been playing all the different versions and variations of them again, and SMAC and Civ 6 are definitely my favorites now, followed closely by Civ 3. Civ 3 held up very well, containing everything I loved about Civ 2 but including elements that are still in modern Civ such as terrain defense bonuses. I still don't take very well to Civ 5, it broke my heart when it was released and that pain is still there. Hexes and one unit per tile were serious changes to the formula, and although I really enjoy those changes now in Civ 6, Civ 6 does everything better, having a more refined gameplay than Civ 5 now that districts are a thing.

On my most recent playthrough, I couldn't believe how let down I was by Civ 4. While it still has the best mods out of the whole series, it's vanilla (with or without expansion packs) gameplay just didn't feel right anymore. I forgot how much of a hassle it was to grow your empire wide in Civ 4, but even doing a tall empire doesn't seem to payoff as much as in Civ 5 or 6 (6 really nails the sweet spot between letting you choose tall or wide).

Now SMAC and SMAX are just pure genius... They hold up incredibly well, still look amazing, even without 16:9 support... but what clinches it for me is the sense of wonder and mystery these games still impose on the player. I played these games for years straight, but haven't gone back to them since the early aughties, so it felt fresh and new. I love the more focused gameplay and the fact that the game actually has a storyline, much like Age of Wonders or some of the great scenarios for the Civ series. Each Culture feels unique, and have their own storylines. Being able to build cities on the water is cool, but getting to design and prototype units is wonderfully satisfying still. I agree so much with most of your points, but you should know that Civ 6 already has released its last expansion pack as far as I'm aware. Only two were planned, so unless that changed recently, I'd recommend grabbing it. It plays much better than Civ 5, and I agree with the most part that Civ 5 looked better, less cartoonish, Civ 6's style really grew on me, and if you want more realistic environments and units that are less colorful, there's an official mod that was released earlier in the summer that gives you the environment terrain of Civ 5 while going with a more subdued color pallette for the units. If you give it a try, I'd be very interested in hearing what you have to say about the game.

My favorite civ game is 2 Test of Time edition.
While unfortunately the advisers fmv were removed, the option to play an extended game where you colonize Alpha Centauri and even fight an interplanetary war, that's just too cool to say no to.

Having played the original through 4, my favorite is 3, which I've been playing now going on 20 years (since about 2001).

I tried Civ 4 last year, but the religious aspect doesn't suit me. Granted, I think I could de-emphasize that within the game, but there was also an absolute game breaker with 4, namely, the fact that no units can ever enter mountain locations. That is simply totally unrealistic.

Now, before anyone gets into a longwinded discussion on whether or not this is realistic, I can tell you the very simple reason why the game was designed that way. It was designed that way to accomodate the AI - period.
They could very well have made it so that perhaps early units would have a hard time entering mountain locations, or perhaps only allowed certain units to enter as time progresses, or whatever. There are many ways this could have been done if the purpose was to treat mountains realistically, except that it would have made it practically impossible to program an AI to properly perform those functions. Instead, they opted to simply make mountains totally impassable, which is easier to program an AI for, but of course is totally unrealistic.
During the short couple of months I had Civ 4 I started numerous games and repeatedly ran into situations where a mountain range - and in some cases a single mountain square - would unrealistically impede my progress for centuries.
I know there's an easy way to quickly enter the map editor and change a mountain location to a hill, thereby making it passable, but sorry, that is not the way I like to play my games.

This is a textbook example of a game being designed to accomodate the AI at the expense of more realistic game play.

I haven't tried 5 or 6.

LOL, sounds serious.
You got me curious, would you like to elaborate?

Nothing quite so serious, per se, but rather one man's molehills are my mountains..

A) The game introduces critical resources. And rather than the multiples that following titles have, you get one, and it can pack its bags, and decide to [expletive] off to the Bahamas. This means that even in a solo game, finding coal can turn into a scavenger hunt where because you don't have coal, you can't build steamships, ergo your fastest option to circumnavigate is molasses.

B) The typical strategy of my choice in any strategy game is to turtle the hell out of it. Civ III practically requires popcorning, which is very much against that strat.

C) In order to get anything terrain related done with any efficiency, you have to throw a ton of workers at it.

Yeah, I would say those are molehills, at least for me. ;-)

According to the playtesters at the time, Civ4 (vanilla, at least) was actually designed with multiplayer in mind.

I like that mountains are impassable terrain. It makes for very fun chokepoints every once in a while (to be fair, more in fan-made scripts like Tectonics than the regular ones). It also works for Mordor-like geography in the Erebus map (from the Fall From Heaven II mod).

But most importantly, the fact that this terrain is almost useless, like ice and to a lesser extent desert, means that other tiles in the map are more valuable. Placing your cities strategically and fighting for the best terrain make Civ 4 a great experience.

Civ5, definitely Venice. You have the freedom of not building settlers and seeking new territory combined with double trade routes and Great Merchant of Venice who can buy city states. It's awesome.