Dwemer Scoped Rifl- Just kidding, I want a crossbow.
Worst? Mannimarco from
Oblivion. Failed on so many levels. Didn't look anything like he did in Daggerfall, did not give the feeling of any sort of awesome power, goes down ridiculously easily, looks like a typical High Elf, and the entire story surrounding him was incredibly stupid and badly written, even by Bethesda standards.
Best? Eh, I guess Mankar gets it. Everyone else is no different from a regular mook, just with more HP to whittle down (or are really just glorified obstacles in Dagon's case). But Mankar gets two flunkies who revive a few seconds after getting killed, making his battle just a little more interesting. But not by much.
Truth be told, best boss in the series I would say is probably Almalexia from
Tribunal. She has a lot of HP, hits hard, has spells that cover the range of the whole chamber where you fight her, and more than anything, [censored] is
fast! You really can't afford to stand still in this fight, making it especially intense and more energetic than nearly anything else Morrowind had to offer. Plus, it's just damn cool when you're fighting a living god wielding Hopefire while using its sister sword Trueflame (and claiming both at the end).
In the end, however, I think Bethesda needs to seriously look at games with awesome boss fights and try to emulate them if they can.
Shadow of the Colossus,
Demon's Souls, those kind of things. I don't really care what form the bosses come in, whether they're human, elf, god,
demon Daedra, dragon, golem , naked Nord, they should be a suitable challenge, and should ideally require a different approach than standard mooks to take down. Thus far, between MW, OB, and FO3, they haven't done particularly decent boss fights.