You mention "rare/exotic lines" but don't give us a budget, so it's a little hard to give you a definitive answer. If you want to produce some really cool Blue Eyed Leucustic stuff, get a Bamboo. If you want to produce some nice recessive stuff, how about a Spotnose Clown or a Leopard Clown? If you want some good base genes to work with, Fire, Leopard, and Enchi all combine with Butter very nicely. Hypo and Orange Dream are two genes that would also combine well with Butter. There are a ton of options out there. A budget, how many females you want to buy, and your own personal taste are the three biggest factors in my opinion.