Have to agree on Excadrill. On my current playthrough i had the same luck as you with obtaining it from the Mystery Egg event. And DAMN! With his decent speed and massive attack as well as access to Earthquake he utterly destroys trainers. He's actually so good that at some times i find myself trying not to use him during certain battles as he would just sweep at least half of the opponents team. Destroyed Call, Kiki and much more.
Would also like to mention other Pokemon that i found useful while playing Reborn.
Donphan - I have mentioned this waaay back somewhere on the forum but i didn't really care too much about this Pokemon until Reborn made me fall in love with it. Nice stats, access to that important EQ and the Sturdy ability helped in every single one of my playthroughs and I always make sure to get one as soon as i can. Also many boss encounters were cheesed thanks to Sturdy and EQ, two examples being PULSE Abra and El's Arceus.
Archeops - Recently tried using it and i was impressed. The defeatist ability doesn't hinder it as much as one would think as it has enough speed to outspeed a LOT of other trainers pokemon. And I've found that the STAB Acrobatics with it's atk stats KO's a whole bunch of stuff.