[quote name='detectiveconan16']I'd rather vote for Arceus, considering I missed out on it because some jerk had to go ruin the event years ago.
I entered the tournament, fought about five or seven times, and lost half my battles using the same party that I used throughout the story.[/QUOTE]
Mhm, Arceus is great and all, but there have been multiple events for the guy and it's pretty easy to pick one up for free (or something small like a starter or fossil poke) on websites like Gamefaq. Aside from that, Arceus doesn't have a unique dreamworld ability.
While there's no possible way to get a Ditto with this ability so far.
*_* I mean, just think about it, somebody setsup two sword dances on their sweeper. You switch in Ditto, and TADA, you already have all the stat mods from the sword dances and you automatically transform, completely skipping your first move.
Ho-oh and Lugia both have pretty awesome Dreamworld abilities though. Wouldn't mind seeing those.