Assassin was maybe the worst class because of the terrible "Shroud" system it used.
The powers are what made the Assassin struggle to function, not Shrouds. Assassin’s Shroud is an excellent idea that was held back by overvaluing of nova damage, and really bad power damage.
And feat taxes that should have just been errata’d into how the feature worked.
But having played both an executioner with shrouds instead of the stock executioner damage mechanic, and a regular assassin with a couple of the shroud feats houserules in as automatic features, yeah it for sure was not shrouds that held the class back.
It still boggles my mind that they didn’t just errata the class. It was a digital only class.
Anyway, “best” isn’t about “power” IMO but rather how fun to play it is.
2e - Bard. Partly because it’s the only class I played in 2e for any significant time.
3/.5e - Rogue, Bard, Ranger.
4e - Warlock, Assassin, Avenger, Rogue, Swordmage, post-essentials Ranger (it wasn’t a ranger until it got magical utility powers), and Bard. Bard is so freaking excellent it isn’t funny in 4e. Oh, and Warden! Seekers are really cool but needed another design pass.
5e - Man…all except cleric?
Some of the most fun I’ve had has been with Rangers and Rogues. I surprisingly don’t really like the 5e Bard much.