Tiers vary from server to server, both in the structure of the tiers and the Pokemon assigned to each one. The following are Reborn's tiers, the structure of which is a fairly common layout:
Ubers- These Pokemon are banned from all others, and are generally regarded as highly over-powered.
OverUsed (OU)- This is considered the tier for standard play. These Pokemon are considered the best, without being too strong for use.
Reborn's tier list can be found here.