Regarding the purpose of this thread: Do you feel OU is the "best"? If so, why so? What do you prefer in OU/UU/NU over other metagames? Do you think that simply not being usable in OU makes a Pokemon bad?
I enjoy OU the most, and have dwelled in UU and Ubers but simply cannot get into them. OU has the most steady source of players, a nice learning curve and it has the coolest Pokes, IMO (ie. Tyranitar>Aggron). Of course, there is no "best". I probably have an attachment to OU since I started in ADV OU, possibly the most balanced Metagame ever.
I intend to play UU sometime before the Generation ends: Many of my favorite Pokemon(Azumarill, Feraligatr, Slowbro, Arcanine and Ambipom to name a few) dwell there, and at the moment the UU metagame is probably the most balanced Metagame there is.
Some Pokemon are completely incapable of succeeding outside of their tier-Corsola is a really cool Poke with no hopes of doing anything, really, and competitively are horrible. They can, however, serve as excellent HM slaves, carrying you across the ocean on its back, moving or smashing large boulders that inhibit your advancement and just looking cool walking
behind you(Remember, slave).
For every "unusable NU" Poke there is something awesome for OU, like Dual-Spiking Nidoqueen, easily setting up your entry hazards and walling fighting types--- even occasionally taking an EarthQuake or two to take out a threat to your team.