With a base attack of 210, all of them are completely unable to compete as psychic-types no matter what so let's look at the secondary typings:
Celebi: Doesn't currently learn any grass moves in Game master, doesn't have available grass fast moves, could get Grass Knot. With any grass fast and Grass Knot it would be a competent grass attacker. With Charge Beam(currently has it in GM)/Grass Knot it doesn't live up to Venusaur/Eggy/Sceptile.
Jirachi: Currently has Doom Desire in the Game master which is a great move. With Confusion/Doom Desire(currently has both in GM) it currently out-DPS's and out-TDO's Metagross.
Mew is a fan favorite but we knew in advance it wasn't going to be great. The TM cost isn't even that relevant because even with optimal moves it's very rarely a good choice for anything.
Tl;dr Jirachi has the most potential and it's not even remotely close. It would be the top attacker of one of its types immediately if released.