I'd have to say the top 5 in order are:
Sonya
Lao
Reptile
Mileena
Raiden
As someone stated before, Sonya can pretty much do whatever she wants online. Cartwheel and dive kick are very abusable online. Her d4 can be easily thrown out without any real danger. Her stupidity is made even safer online.
Lao's pressure and teleport are amplified online. It's much harder to go for big punishes on teleport and you're pretty much reduced to an uppercut or dashing forward. His crossover and jump-in pressure is also very annoying to deal with online.
Reptile's elbow dash is harder to punish (I can really only punish it with Cage's uppercut or a throw) and like Mileena, his d4 is abusable as well. He can dominate the elbow dash distance much more easily online.
It's much harder to play footsies with Mileena as d4 is easily abusable. It's much harder to react to random rolls and EX teleport can be pretty difficult to punish properly.
Raiden's teleport is really annoying to deal with sometimes. Most flowchart Raidens love to jump as soon as they teleport so I almost always NJP as soon as I see a teleport.
Some honorable mentions:
Kitana (d1 is way more abusable, much easier to play a lame style, easier to get hit by her big combos)
Liu Kang (his pressure is much harder to get out of; sometimes I can't even counterpoke)
Cage (more annoying to get out of his pressure)
Sindel (d4, cartwheel, and her fireballs can be annoying as hell to deal with)
