I would again repeat that there is no BEST DESIGN for multihulls. It really depends on what you want to do with the boat.
For instance, if you're really into going fast... a Reynolds 33
catamaran might be a very good choice. However, for about the same price, $150,000, you could get much more comfortable cruising catamaran, like the Gemini 105Mc.
Both of these catamarans are relatively narrow, having a beam of only 14' and both will fit into a 40' monohull slip. A different sort of catamaran would be a Seawind 1000
, which is almost 20' wide and generally requires a mooring or end slip, and has fixed berths for EIGHT...but is probably considerably more money than the other two.
The same is basically true of the trimarans. The go-fast version would be a Corsair 28
. A basic cruising trailerable trimaran would be my boat, the Telstar 28
. The luxury cruising model would probably be the Dragonfly 900
, which is probably almost double the price of the Telstar 28.
Your budget is going to have a very serious impact on what boats are possibilities... and what ones are just pipe dreams. Going above 35' LOA starts to get really expensive, really fast. A new 40' Nautitech Catamaran goes for about $430,000.