Tournaments I usually will play in have banned Sonic from the roster, as well as smash balls. The reason is simple actually. Sonic is an absolute cheap shot, and in reality should NOT have been added in to Brawl. Want a good replacement character? I think anyone would be better than Sonic as a brawl character, because they just made him too powerful.
My hatred of Sonic actually stems to a tournament that was held at my school in which some jerk claimed it to be hosted by the club I was vice president in. He allowed Smash Balls and he entered the tournament (which are two taboos right there. If you ORGANIZE a tournament, you should NOT be entering the fucking tournament because if you win, you have everyone hating the GROUP that was said to have organized it, not the INDIVIDUAL who lied and actually did organize it...plus he cheated the club out of $50) I enter tournaments with friends, and I can usually get to the Semi Finals rounds using either Marth, Ike, or Captain Falcon.
Marth method: Counter Attack abuse. Marth has good speed, though it's fairly nerfed compared to his Brawl self. It's not too difficult to use Counter Attack, just don't spam it. I beat the guy who was using Sonic all the time with smash balls active by using a Dancing Blade combo, juggling him in the air til his damage was over 200% and then the smashball shows up. He gets it and I kill his ass, taking the ball and killing him the second his invincible wears off.
Ike Method: Ike is a powerhouse. He may be slow, but his attacks are vicious. Though in all honesty it feels a bit like choosing M.Bison or Akuma in Street Fighter, a character whose power is only in his speed.
Captain Falcon Method: This is actually a gag for me. I love to falcon punch people out of the arena, especially doing last-second kills by falcon punching them form the edge, then when they try to come back, jump from the edge, falcon punch them OFF the screen, and the timer running out before I hit the bottom barrier...