Well it depends. You can either go with recessive traits like blizzard, albino, leucistic, or you can go with traits like pastel, super/hypo, tangerine, carrot tail etc. Recessive traits tend to go for less because leos breed so fast, and it would definitely be worth getting two of the same trait (no hets) to breed, because hets are worht almost the same as normals. You might want to get into double hets like patternless albinos. The double hets are fairly cheap, get a pair and you can produce some patternless albinos which go for a lot.
If you end up going with snows, pastels, hypos etc, then you'll probably need to fork out a bit of money to get a pair of snows or super hypos or whatever, and some babies you end up with will probably be nice normals, but many will be super hypos which still go a lot because they keep getting improved.
Just go with what you like!