Street Fighter 4 is by far the hardest. The AI characters are relentless in comparison to the older games, and the basic game pads (360 and PS3) are terrible for the game.
What you mean to do:
shoryuken:
Right, bottom left, bottom right, punch.
What happens most of the time:
Hadoken:
Down, bottom right, right, punch.
I'd like to point out that this is not a button masher and issues like this can really screw your game.
The keyboard will be allot easier to use. But I'd always recommend a popper arcade stick.
And charge
is easy

All you do is hold a direction for about 2 seconds before continuing with the rest of the move.
And about Ryu, Ken, and Akuma. Really, really not the case. Sagat, Chun-Li, Gen, Balrog (or Bison if you go by the original Japanese names),M.Bison (Vega), and Vega (Bison), are all heavily used.