@Sasuke: Your sig is making these forums break... please make it smaller for the few people in here that don't trust the autosize function :P
@Topic: My favourite Boss'd be... well, The End from MGS3... I got bored the first times I played that, but if you play it the way Snake would (no health cheating for The End before by shooting his wheelchair, no special camo, no thermo goggles), you can get some exiting hours... and annoy your parents with "Goddamn, can't you see my life is in danger right now?" :D
Well, Psycho Mantis is great, though I only know the NGC version, as stated... I was really surprised by him telling me I like SSBM... I mean, what a jerk... I love SSBM! Loser... ^_^
It's a little harder to find a good boss in MGS2, because they simply can't stand the duel with the MGS3 ones I like best 'til now... I guess I like Solidus best (the blade duel), even though Raiden is one of the few guys who absolutely can't handle a blade ^_^ Obviously, he improved 'til MGS4 ^_^ Well, for Solidus... I still have to beat him on Extreme, but I cimply can't beat the breath holding game -.- Solidus is a bitch...
Lastly, for those who now the GB episode, my favourite boss is Viper, the last boss. First, you had to shoot down Metal Gear's Legs so he can't move anymore. Afterwards,Viper jumps off and you had to defeat him by placing claymores on places where you'd think he'd walk over while you had a time limit. It is a damn hard battle every MGS fand should've experienced...