It changes from game to game, also it depends if it's about design or gameplay, for example i like Chun, Cammy, Guile, Gief, and Sagat as character design but never played those characters much. So for each game characters that i used the most:
SF2:
1- Ken
2- Ryu
3- Fei Long
SF Alpha: (barely play it)
SF3: Dudley
SF4: Dictator
SF5: Start with Laura and Nash, then when Boxer came out it was Laura and Boxer all the way until... Poison came out, and since then it's Poison only