Aside from the obvious, Mewtwo?
Based on smogon usage stats for November:
We'll go with Pokemon that have at least a 5% usage in Ubers, as anything below that can't really be considered anything you encounter often, and as such can't be considered a "main" Psychic Pokemon.
Lugia: 18.78% usage
Deoxys-A: 12.141% usage
Deoxys-S: 11.883% usage
Latias: 7.063% usage
Latios: 5.826% usage
BW UBER: Lugia, Deoxys (All formes except maybe defense), and to a lesser extent, Arceus-Psychic.
DPP UBER: Above, Wobbuffet, Latios and Latias w/ soul dew, Jirachi.