Personally I think the best medium sized snake is the Dumeril's boa:

here's my little girl:



Dumeril's don't get quite as big as RTB's do but still are still a nice step up from a BP. Dumeril's are very calm and docile snakes that are great to handle. They spend most of the time out in the open (partially buried if possible) They are great feeders and are beautiful. I think if you are going to get a medium sized snake as your first snake a Dumeril's Boa is the best choice, they grow slowly and really are a joy to work with.

Red tail Boas are great as well, they are a little more active than Dumeril's but can also get really big.

Bloods are really cool snakes, and I hope to get into them in the future, but they can have a pretty nasty attitude.