@flareblitz: imo the hardest level in radiant dawn is either chapter 4-3, distortions or chapter 4-4, revelations on the hardest difficulty, idr which. well it actually might not be: i just remember looking at the map before making any moves and thinking "oh fuck that" but decided to give it a go anyway, and once again, a few turns in i thought "fuck that" and gave up. i'm pretty bad at fire emblem though, i wish i wasn't.
@berserker lord's post about wily's castle: i wholeheartedly agree with you.
thinking about it now, i haven't played many games with difficult bosses. most of the bosses i've encountered are in the zelda games and gyorg from majora's mask was the hardest that i can remember (still not that hard really, so long as you have a bottled fairy) although, now that i think about it, oot ganon was actually pretty tricky, it's just the last time i battled gyorg was about 4 days ago and the last time i battled oot ganon was about 4 years ago so...
a few that stand out though:
if anyone loves tales of symphonia as much as me, they'll know that there are multiple paths through the game- towards the start you can just ignore all the events that occur at palmacosta and do... various other things instead, and then go back and have an easy-as-hell time with magnius and pronyma's servant thing that you fight in the basement of general dorr's house.
so on one of my playthroughs, i found myself fighting iapyx as the second boss (well it depends what you count as bosses but basically i did the triet ruins, and then pretty much went straight to balacruf mausoleum). it was a while ago now so i don't remember how i did it but needless to say i had a tough time with iapyx and the sheena fight that cooper mentioned.
other noteworthy boss battles in ToS are that one where you play as sheena and fight kuchinawa by yourself, i had to look up a strategy for that on youtube, as well as some of the colosseum fights, in particular the one that is made up of cameos from some other tales game, and the one where you fight seles.
also the final boss of no more heroes 2 (has anyone played that?) on the hardest difficulty is tough, basically because you pretty much can't use the peony, so you can't do as much damage as would be ideal, it's difficult to get an attack in because he attacks so fast, and many of his attacks will just smash you out the window, killing you instantly. in no more heroes you have a hell of a lot more health than in the average game so "insta-kill" is a big deal.
aaand finally the last boss on viewtiful joe on the hardest difficulty. i don't remember much about this fight other than it was hard.
oh and solace: taboo sho really isn't that hard if you have the right pins (LIGHTNING RAGE) although that can be said for most bosses in that game i guess
