Personally one interesting boss for me was [spoiler=Earthbound HUGE Spoiler; final boss spoiler that is]Giygas. I already somewhat knew that it was "Use a special technique" but I had no idea it was Prayer so I kept having to fool around. It's also...Well...Check a video on it on youtube or something.[/spoiler]
As for a long boss [spoiler=Mario & Luigi Bowser's inside Story]Not gonna lie here; I beat it my first time but I was so underleveled and had a bit of an annoying time with the fawful phases + Wasn't all the greatest at bro attacks[/spoiler]
So how bout you?
Easiest boss ever: Yu Yevon from Final Fantasy X. Unless you willingly petrify everyone on your party, it's impossible to lose.
Hardest Boss Ever: Queen Larsa? Dunno, any boss I find hard becomes easy once I beat them.
Interesting: Any of the Colossi from Shadow of the Colossus.
Easy: Yu Yevon, probably. The first battle against Lord Crump at the start of TTYD is also unloseable. Easiest boss that you can actually lose against? Probably Monstar in Paper Mario.
Difficult: Margaret in Persona 4 on Expert difficulty. If we exclude bonus bosses, Vitreous or Trinexx from Zelda: ALTTP.
MOST ANNOYING FUCKING BOSS EVAR!11!11: MOLDORM AAAAAAAAAAAA
Most Fun: Luxord in Kingdom Hearts 2.
in b4 dullahan or culex w/o lazy shell
OCTO-HEAVY
Other than silly TF2 things, pretty much every boss in KH was amazing.