I have been in an adventure game phase recently and wanted to give some of the classic Sierra adventure games a try. Now I know that many of them are frustrating to play nowadays because of cryptic puzzles and dead ends.

I dont mind hard puzzle but dead ends make me rage quit. What are some Sierra classics where I dont have to follow a walkthrough perfectly or else I cannot beat the game?

I finished King's Quest VI without a walkthrough (no internet back then). I don't remember any dead ends. Sudden Death because you click the wrong pixel, on the other end, there was a few, and also wrong timing and millisecond during the "real time" sequences of the game.

The later ones, I'd say. Roleplayingy stuff like Hero's Quest / So you want to be a hero. Or Conquests of Camelot, Space Quest 3, etc. The Leisure Suit Larry weren't too crazy either, even the first one was quite tame. I think the Police Quest ones were also pretty do-able, if you really followed the manual's procedures. Colonel's Bequest is an odd one, it's really about exploring "dead ends". You wouldn't complete it in one playthrough, but its investigation logic is all about experimenting being in different places at a given hour - so, re-playing it multiple times until you get a sense of every characters' schedules (Sexy-Brutale-like, in a way).

The early Space Quest were really sadistic. Sierra mellowed a bit with time.

(Edit: The wonderful Manhunter:NY and its sequel are quite easy as well, but I don't think GOG has them, does it ?)

Is the problem that younger generations expect instant gratification and aren't being encouraged to overcome problems by applying renewed and continued effort? There is a point where you reach a dead end and rely on walkthroughs but figuring it out yourself is rewarding.
Dummying down these types of games is something I don't want to see. Giving participation trophies to everyone and refusing to distinguish achievers is a disturbing trend.

Dynamix which produced Betrayal at Krondor was part of Sierra.
Your group can die, and it is permadeath.
When I first experienced this, I laughed. I didn't rage. I started over and learned from my error.

Obviously it is very subjective but I would say that the newer the sierra adventure, the more straightforward and not always that means a bad thing.
Because Sierra adventures used to have the sin of the social media of the era, the hints manuals and the phone line hints to complete the game. Pretty unfair in the bad sese :)

Anyway if you want something fairer I'd recommend later nineties titles. And believe me, they are not a piece of cake. But at least they are not outdated evilness or random deaths titles.The worst you can suffer is hard puzzles, but that's the idea indeed.

Freddy Pharkas
Leisure Suit Larry 6-7
Kihg's Quest 7-8
Torin's Passage
Gabriel Knight series
Lighthouse
Shivers
Phantasmagoria
RAMA
Space Quest 6

Greeting

This is true to an extent, but Sierra (especially early Sierra) had some pretty daft moon-logic and lots of dead ends that were most likely done to make the games feel longer and more worth your $50. That's not really something we want today, with endless games to play.

I thought those dead ends were bullshit then as I do now. I never understood what was supposed to be so great about the old/early Sierra adventure games other than being the new hotness at the time.

Ah the good old Sierra classics!

King's Quest
I'll repeat the earlier recommendation for King's Quest 6. It's likely the high point of the series and was very well designed with multiple endings. King's Quest 7 seemed to fall apart in time/budget close to the end but is an overall OK experience. 8.... well that is a pretty different experience and most people don't find it a good one.

Space Quest
The humor in these games tend to make them fairly fun to play overall. 1 & 2 have some annoying dead ends, which while amusing will make you reload an earlier save. Or restart the game. 3,4,5 are solid entries in my opinion. 6 wasn't my personal favorite and does have a particularly annoying puzzle that requires using the included documentation.

Police Quest
Alas, I've only ever got around to playing and finishing 1 & 4. I was able to finish 4 without any extra hints though, if I recall correctly.

Quest For Glory
Really enjoyed these ones, They're basically the standard Sierra games with RPG elements added on. You can take a character from the first game all the way to the fifth if you so desire (You can skip games though). The fourth game was amazing for it's time and is still one of my favorite Sierra games. I believe there are some fan patches available that get rid of most of the bugs for the third and fourth games now too. There's also a very well done fan remake of the 2nd game which I think is compatible with the import/export process.

Gabriel Knight
Played and finished the first, but did need some help as there are some easily missed items. Fantastic writing and plot for it's time.

I'm pretty sure Sierra themselves once admitted the real purpose of so many random cheap deaths, dead ends, etc, was to sell hintbooks / encourage the use of premium phone helplines (209 683-6858 is even advertised both in-game and in the printed manual). At one point they claimed they sold more hintbooks of Leisure Suit Larry than they manufactured legal copies of the game...

Oh, and a couple of games I forgot to mention because they are indeed understimated are the Conquest of Camelot and Conquest of the Longbow. I believe they fit very well with the OP wishes. In fact they are forgotten classics, very well designed, solid and fair games.

And of course, as has been mentioned before, the Quest for Glory series are a must have

I think you misunderstood the OP. By "dead end" they don't mean getting stuck because you can't figure out a certain puzzle solution on your own, they mean screwing up your entire playthrough without the game telling you, so you only notice it hours later and have to start from scratch, or from a savegame way way back. And repeating everything you've already done before isn't exactly entertaining in story-driven point-and-click adventures with fixed puzzle solution and more or less linear structure. It's perfectly reasonable to not want to waste your time on that anymore, when it was annoying even back then already. As an example, the game would allow you to, let's say, eat an egg you pick up, then require you to use it much, much later for something else, or you can't continue. Things like that actually exist in some Sierra titles and even in the earliest LucasArts adventure games, before they came up with the "you can't fail, so feel free to experiment" concept that is the standard for adventure games now.

Indeed. I do not mind moon logic in a point and click adventure, I do not mind very hard puzzles, in fact I demand it, I do not mind deaths if they make sense and/or it is fair into the flow of the game.

Getting stuck in a game is not a dead end, a dead end was a lot of times something the original designers overlooked, design errors.

^This.

For instance, dead ends in Maniac Mansion:

https://gaming.stackexchange.com/questions/207693/what-are-the-dead-ends

A dead end in those cases is a situation where you lost the game but don't know it, so you'll keep trying to no avail.

No, the problem here is that Sierra games are quite simply badly made. Not everyone has the sad amount of time on their hands to restart as much as you do.