In your opinion, what is the best class per edition and why is it Bard?
I helped!!! Helping!!!
No but more seriously, talking mostly about being powerful and impactful:
2E D&D - This is a tricky one and very dependent on what classes are available, and whether we including multiclass combos, and what rules your group followed/didn't follow on MC combos. Also depends on how high level you're going to get.
I think the most outrageous strictly-legal combo I ever saw in D&D was a
Speciality Priest of Clangeddin/Fighter, which had a kit which allowed Weapon Specialization. This was upsetting but absolutely legal - SP of Clangeddin, specifically allows both multiclassing to Fighter AND picking a kit so long as it's allowed for F/C combos. The obvious kit allows Weapon Specialization, normally disallowed to MC characters (not that anyone followed that rule).
Generally speaking I'd say
Bard because their XP table was ridiculous to the point where they got 5th-level spells sooner than a F/M did (on the same XP), and they were a fantastically multitalented class.
But if you were going to get to high levels,
Specialist Wizard was the best -
Transmuter particularly was incredible.
Shout out to
Psionicists if allowed for doing some wacky but weirdly well-balanced stuff.
Special
Worst Class award to 2E
Mystic, which had misfortunate of stupidly playing once, which was basically a semi-pacifist hippy who was bad at everything except making low-grade magic candles.
3E -
Wizard PrC'd into one of a number of outrageous PrCs that gave you all your normal spell advancement and a bunch of other good stuff too.
CoDZilla also an excellent choice.
4E - Very hard to say because 4E was very well-balanced (sacrifices were made to achieve this of course), but I'd personally say, if we're just talking one class, maybe
Battlerage Vigor Fighter? They were absolutely STAGGERINGLY hard to kill. The sheer amount of HP/THP they were coming up with was just mind-boggling, and their damage output was pretty significant.
Avenger, Battlemind, Invoker were also all great classes. Vampire was a mediocre class but fascinatingly implemented. Assassin was maybe the worst class because of the terrible "Shroud" system it used.
5E - Solo all-rounder? Probably
Fiend Warlock (Pact of the Tome). Hard to put down, high CHA for social situations, and does outrageous damage, even gets to light of a lot of upcast Fireballs, and Tome gives gives ritual casting and a bunch more cantrips, particularly great for a Warlock. In a well-run party? I actually rate that over Hexblade for survivability and damage. It's almost "any Full Caster", but probably
Diviner Wizard or something.
Twilight Cleric is definitely strong too. Lore Bards are good but probably not as good as those. I could also see
Ancients Paladin.