In no particular order:

Flay Allster - What a bag of douche she was, and she wasn't even that good at it so I couldn't respect her for being a magnificent bitch like I did Rau LaCruset.

Hitagi Senjougahara (Bakemonogatari) and most other 'modern' tsundere chars - The reason here is because I thought she was an emotionally weak, bitchy character like most 'modern' tsundere chars seem to bee. I don't usually have a problem with emotionally weak characters who are aware of and open about their weakness. I also don't have a problem with bitchy characters who can back their bitchiness up with some tangible emotional strength that demonstrates that they really don't need or care for the approval of other people. However, 'modern' tsundere seem to glorify the idea of being a bitch because one can't or won't treat people with respect due to some personal, emotional reason while still craving the approval and support of others (or a specific other)'. My dislike for this character type is boosted to hatred by the fact that characters like this are often gifted with a love interest who is infinitely willing to suffer their bullshit so they don't have much impetus to change their bitchy ways in order to fulfill their emotional support desires.

Cecily Cambell - Similar to the above, my hate and those like her comes from her vapid sense of entitlement and self-righteousness which is backed by nothing but her superficial title and her personal belief in her convictions. Cecily's character is introduced with nothing to her character but convictions yet time and again she tries to behave as if her convictions alone are enough to demand that the world bend to her will in the service of said convictions. She repeatedly need to be rescued from the consequences of her folly by other characters. To make matters worse she has shown a tendency to turn around and condemn those who have save her from herself in the past, once again based on nothing but her self-important belief in her convictions.

Akira Takizawa (Eden of the East): I just never liked this guy. Something about his personality pissed me off immensely nearly every time I saw him say or do just about anything after a few episodes. I could respect him somewhat for being decisive and bold at times but his overall attitude just grated on my nerves for some reason and I couldn't stand him.

Repeats:
Sakura from Naruto - Not so much for being mostly useless in battle except that one time, but for being a self-absorbed ninny who thinks everything should revolve around her love life.

Marina Ismail - I give Relena a break as she seemed to grow somewhat over the course of her series and at least tried to make a real difference a few times. Marina was just a completely useless character who did nothing but get in the way and try to undermine the determination of people who actually tried to make things better.