Agree with vagabond: Tier is relative, an example would be Dhalsim from SFIV who is generally rank amongst the mid to lower tier character but was used to win the Japan National SFIV tourney (Iyo beat RF who was using 'Mr OP' himself: Sagat).
Tier also changes as players find new ways to exploit and use a character to their maximum potential. There are some cases were tier REALLY do exist in a bad way, making some matchup and indeed some character unplayable in tournament condition such as Rock from Soul Calibur IV but in general most character can be used competitively to win tourney even against equally skilled players, in the end, the key to most fighter games (aside from sound knowledge base) revolves around reflex, anticipation and a small amount of luck.
