When I got the game I mained Sagat. Eventually got bored with him, as he's very much a one trick pony, especially when playing on-line. I've now moved onto Rose and C-Viper and usually alternate between the two.
Rose is actually better than most give her credit for. She has a wide range of tools, and I find most online players still aren't familiar with what she can do as she's probably the least used character in the game. Her only glaring weakness is that she can't do a proper reversal without burning EX meter.
Viper's definitely the most impressive character in the game. Trouble is my execution isn't anywhere near good enough to play her consistently. My brain thinks the right thing, but I can't my hands to obey.... Even her b'n'b combos like EX seismo, HJC, Burning Kick are hard to nail everytime. Although on the rare occasions when I do get into a groove, she is hard to beat.
When I played Sagat, I could maintain a win rate of around 65%. Now with Rose and Viper it's down to 40% and falling all the time...