In support of Jirachi, I'll go ahead and throw down some more notable things about it-
-TrickRachi is apparently a nifty way to run with it.
-Just it's overall typing coupled with it's defenses. Goes hand in hand with steel types.
-Strong support moves (Dual Screen / Wish / Rain Dance / Toxic / Thunderwave [No one uses anyways, but it's there])
Although there is no strongest Pokemon, because x < y in every case, but the solid movepool Jirachi offfers, the ability to either hit on the physical/special side, and strong defenses make it quite the heavy contender.