Time Eater from Sonic Generations. Boost to win much?
That guy took me a while to beat. those rings were REALLY hard to see at times. And at times, it was impossible to get close like he was rubberbanding.
But it wasn't annoying to me, just long.
The boost guardian wasn't too annoying for me either. Sure I could nearly die, but it never gave me too much trouble. Surprised the boost guardian wasn't up for metroid prime 2.
For annoying... Nobody made me say "JUST DIE ALREADY!!!!!' More than Mistral from metal gear rising. I like her theme song, i'm the kind of guy that will make boss battles last longer just to hear a theme song (Some times i'll hide from metroid prime space pirates just to hear their theme, or prolong the Rundas fight in metroid prime 3)
But Mistral and those FUCKING BALL THINGS!!!!!!! I HATED THEM IN METAL GEAR SOLID 4 AND I HATE THEM HERE!!!!!!!! She's not hard, those ball things just kept getting in my way.
My second nomination is the Metroid prime 3 hypermode playthrough stopper, Mogenar. He wouldn't be annoying if you just had ONE MORE HEALTH BAR!!!!!!!!!! I then found it was luck. The time I beat him is the time he was doing that one attack that gives you health more often.
My third nomination, soul calibur 5 nightmare. Not the story mode boss, but the rank 1 nightmare in quick play. I literally stayed up all night trying to beat him and when I did, I woke up my friend XD He's just so fast and OP. I was trying everything to beat him with all my best characters. Siegfried, Nightmare, Mitsurugi, but I FINALLY beat him with Natsu.