In addition, the tier a Pokémon is in doesn't necessarily denote how good it is. For instance, Electivire was OU back in Gen 4 despite being awful and Mega Heracross was BL for a while despite being extremely good in OU. I'd recommend a visit to the Viability Rankings thread of each tier to get a better idea of how good a Pokémon is.
Oh, and before I forget, welcome to smogon :)
Oh, and before I forget, welcome to smogon :)