Dominion has come a long way since the Mountebank/Chapel days, what with Events, Shelters, Heirlooms, Baker etc. Here's some of the things you can do now:
-Open Platinum, with 4 Coppers and a Cursed Gold and Baker and Borrow.
-Activate Magic Lamp on turn 1, with 4 Heirlooms and a Copper and Villa.
-Gain a Mercenary before your first shuffle, by paying $8 (using any subset of the things from the Platinum opening) for a Traveling Fair and 2 topdecked Urchins.
-A bit more mundane than the others, but still really good: Summon Ambassador on turn 1, removing 2 dead cards and giving your opponent one before the first shuffle.
I doubt we'll reach agreement on what the very best opening of all is, but what are some of the craziest openings you can think of?