Appearance wise, the galleon by a country mile. As far as playability and fun, the sloop wins hands down for me. It's just so much easier to co-ordinate one other player and get everything done right in a sloop than it is on a galleon. On the galleon there just seems to be an attitude amongst many players that someone else will get it done, or they'll go afk, even for a short time - and ofc, that's when a meg or a ghost ship will come at you - or other players lol.
The brig just isn't fun for me at all. The lower deck is cramped and when you're on the wheel, the thing just looks like a banana boat the way the deck curves lol.
The rowboat though... that thing is worth its weight in gold. So many bad encounters saved due to a rowboat to collect all the loot in whilst the fresh ship sails back to you. Almost always Karen's fault :P