I hate DRM and so far I love this site. That said, the vast majority of these "classic games" hold no nostalgic value for me, since I didn't even own a PC when they came out (or was too young to care). So now I'm overwhelmed looking at the catalogue of good old games and honestly don't know what to get. So without knowing my personal preferences in games (still figuring out myself), are there any games you'd recommend? In other words, what games should pretty much anyone try out?

Sorting GOG by bestselling makes me think the Baulder Gate's and fallout series, along with Planescape are the cream of the crop, but "bestselling" probably fluctuates, so I wanted to make sure.

Those are a good start but they are all RPG games. If you want some variety I'd recommend the Legacy of Kain series starting with Soul Reaver 1 (fast 3rd person action). If you want shallow fun racing Flatout is a must have. Empire Earth is probably the best RTS (in my opinion) on GOG. The the Commandos series which really don't have a genre but rather appeal to people like me who have seen "The Guns of Navarone" more times than should be legally allowed.

Lots of other great games too, but I agree with triock and TinyE on these two and also;

Alpha Centauri.
Syberia.
Deus Ex.
Planescape: Torment.

One of my favorite gameseries is the Little Big Adventure series. Pretty hard at times, but the look and feel of the games are one of a kind. I especially enjoy the first one :)

In no particular order:

-Sacrifice
-Planescape: Torment
-King of Dragon Pass
-Baldur's Gate 1 and 2
-Giants Citizen Kabuto
-Zeus+Poseidon
-Painkiller
-Eador Genesis
-Heroes of Might and Magic 3
-Unreal: Gold
-Postal 2
-MDK
-Gothic 1 and 2
-The Witcher
-Unreal Tournament 2004
-Carmageddon

This is just part of the best games here on GOG, play these and then you can look at the rest.

It actually depends quite heavily on what you are looking for, and how well you can tolerate the older graphics.
For RPGs, Planescape Torment and Ultima 7 would get my vote.
For Adventures, Quest for Glory series, which is quite unique on their own.
For 4x, Master of Magic, hands down
For tactical combat, Jagged Alliance 2
For RTS, Evil Genius, for again a unique take on the genre, even if it's not what most people would call an RTS
Carmageddon is another must as well.

There are tons of good and excellent games here, and I can probably make a "must play" case for at least 100 of them, with the rest of the community being able to make a case for the other 400 (yes, I've seen a case been made as to why one should play MoO3).
So, without knowing a bit more about what you'd like to play, be prepared to see a lot of the games suggested.

I'd recommend:

Sanitarium
Duke Nukem 3d
Spellforce 2 shadow wars
Crusader no remorse
The longest journey

i would suggest the last express

Deus Ex

Planescape

Fallout 1

Fallout 2

Duke Nukem 3d

I do have my own personal favorites, but I know that not many might share the same affection for them.

Lol thats what I expect... but this should help narrow it down slightly at least. As for tolerating older graphics, Im much more interested in whether or not its a good game. Im a bit skeptical of an early 1980s game being as "fun" as an early 90s games, because technical limitations can certainly effect gameplay. That said, Im sure there are plenty of exceptions, and as long as the gameplay hold up, Im not worried.

Yeah, tell us what games you like, what genres you enjoy etc., otherwise this is just going to turn into a massive 'favourite games' list. Also do keep in mind that people here very much are nostalgic, so our view of these games could be slightly ... Non-objective.

You have always been one of the nicest and most gracious people in this forum so it hurts me to say this but, Postal 2?

Postal 2?

Really?

A game called "Postal" which you can actually win without killing anyone is not a game, it's a fraud! Postal 1, now there is a classic!

You are going to get so many recommendations they'll be more or less useless, just saying. However, here are a few of mine:
Sacrifice (3rd person RTS, sort of)
Giants: Citizen Kabuto (1st/3rd person shooter with over-the-top humor)
Dungeon Keeper 1+2 (RTS)
Heroes of Might and Magic 2 (TBS)
Settlers 2 (RTS/Simulation)
Sanitarium (Adventure)
Hostile Waters: Antaeus Rising (RTS)
Unreal Tournament 2004 (FPS) Now, I'm not trying to be patronizing, I'm genuinely curious. Is there any kind of explanation for why so many Americans in particular seem to insist on writing "Baulder" instead of "Baldur"? I see it all the time, and it always seems to be Americans doing it. I've even wondered if the title of the game was spelled differently in the US release, but that doesn't seem to be the case. I'd put it down to typos or spelling errors, except that it seems to be always spelled wrongly in exactly the same way.

King's Quest VI
Deus Ex
Baldur's Gate 2
Fallout
Thief 2
Gothic 2

That's a PC gaming classics list if ever there was one, if you ask me. Of course others will disagree. However I tried to list classics, not personal favorites (I prefer many RPGs to Baldur's Gate, for example).