Picking Mew because it can learn Transform, or because it can learn every TM, is a pretty pointless reason. Transform is a terrible move, and the TM capability does it little good, because it doesn't get any STAB bonus off of anything other than Psychic-type moves. I guess it's useful for HM slave purposes, or you can teach a different Pokemon Fly (that being Mew) without actually having a Flying-type on your roster, but Fly is such a bad move that it's basically a waste of space.
I like Mew, but Mewtwo is a superior Pokemon on many levels. The only thing Mewtwo doesn't have over Mew is the TM capability, and the level of rarity (I won't even include Transform as a reason, because again, that move really sucks).