OK, this is moving to the off-topic realm, so we better make decisions:
1. I use the monkey myself, but since FLAC has been hugely popular over at HA (the latest poll), I'd rather go with it too. It'll make life easier for those who wish to submit samples, since it's more of a familiar codec.
I'll create a poll, the results of which will not dictate to Ganymed which codec to use, but if enough users support guru's opinion, Ganymed and I will seriously consider switching to another format.
Vote to show us what you want. Since HA users are very much involved with this, and the database is not only for MPC users, I suggest that one of you duplicates the poll on HA, so that many more have the chance to vote.