I'm going to spend some time in the very near future with a pair of Merc 400's. I'm looking forward to getting to know them as I think that's a bit of a game changer, at least until the 500's come out.

If you like car boat engines, you could always opt for a pair of Seven Marines with the Caddy engines. Personally I think they're a bit of a novelty as the weight and cost are prohibitive, but they do look and sound cool.