Most annoying... Sonic Adventure. DAMN YOU EGG VIPER! DAMN YOU! (that one boss is VERY annoying) Sonic 3 alone comes at a CLOSE second (DAMN YOU FINAL BOSS!)
Shinobi on the ps2. Hands down the hardest game I've ever played. I was fighting the box where your on a platform surrounded by lava I believe. In case you've never played it, you really didn't do any damage to bosses unless you kill enemies in succession building the swords the strength. I hit her once after my sword was strong, which put her down to about one hit left before she died. and with full health, she had one hit left before she died. This was my 11th time trying to fight her. excited, I went to finish her off,and she managed to kill me some how. I had full hp, she had a sliver of life, and I lost. I was so angry I caught my ps2 with a well placed left hook, immediately ending the shinobi disk's life. I had to completely dissemble my ps2 and fix it to get it working again. a furious day indeed.
Hardest game, but still fair? I'd say Ninja Gaiden on Xbox.
Hardest game, and unfair (and subsequently annoying)? I'd say Quake 3 arena if you're fighting against the bots on the hardest difficulty level. They cheat. No - really, they actually cheat; ID software indicated as such in an interview =P
Okay so I read all EIGHT pages of this thread and not a single one of you has mention Battletoads, especially if your playing with a partner! If your partner hit's you you lose some life, You and your partner share lives, and it's freakin hard! Remeber the motorbike level? Yeah touch anything at all, you die. And you need to have at least four lives to make through that section! Oh yeah if your partner touches you you move, you hit something, you die. I have said it once and I'll post it again it is the only game at the time that made you want to break your controller, SNES, your face, your partner's face, the TV, your partners controller, and the face of anyone unfortuante enough to be a spectator. I say if you want a real challenge don't look for ninja gaiden for Xbox where you can SAVE. Look for SNES super hard games where saving would have been the equivalent to someone curing you from cancer! If I had a saving system in battletoads, I would have died, came back, and played again! That is all.