Probably the 4e Assassin, in spite of it's power issues.
At-will teleportation, stealth, shadow magic, stabby-stabs. Everything i want in a dnd character. Executioner with some original Assassin powers and stuff almost fixes the class problems, but...man...I wish they'd given the class an overhaul like they did for the warlock. The Assassin/Avenger/Covenant Agent build is so tasty. I just wanna play a holy avenger of the night who teleports and whose blade is guided by faith.
4e Warlock/Hexblade comes close, as does the 4e Bard.
The 3.5 Bard is one of the only things I like about 3.5, and I still miss how it's songs worked, and wish they'd been updated to 5e as Bonus Action abilities in the base class, or maybe BA songs if necessary.
None of the stuff I love in 5e is quite so "absolutely this" for me as the above 3, but I really do love most of the classes. I've even come around on the Fighter, a bit, though mostly as a multiclass that makes a ton of character concepts work better. I doubt I'd ever play a Fighter straight for more than 5 levels, though.
I love the concepts and ideas of the 3.5 Ranger, and the late game 4e Ranger really got close to fulfilling the promise of those ideas. The 5e Revised Ranger also gets really close, but it's hard to ignore all the features that come online like...10 levels too late, and/or do way too little for the level you get them.
I really wish sometimes we could have 4e classes on a 5e chassis. I really do prefer the core engine of 5e, for the most part. But good gods above letting each class concept breath and stretch it's wings did wonders in 4e. You can't get the Avenger as a subclass in 5e. You can get a thing that maybe reminds you of it a bit, a few fights out of the day, if you ignore the armor and how your attacks are more about muscle and training than faith and relentless focus. Or if you wildly reflavor the Hexblade Warlock, I guess.
Nothing, not even a very effective shadow monk/gloomstalker ranger build or something, even at high levels, achieves the same thing as the 4e Assassin, and nothing will because subclasses just aren't big enough to do stuff like that.
Anyway, overall without regard to specific edition, my top 5, kinda in order probably, are;
Assassin
Bard
Ranger
Warlock
The Vague Class Concept That Isn't Always Fulfilled That We Call Gish