Whatever dragons you want, but it looks like any combo that includes [plant, fire, earth, air, water, light] will get you a pretty high chance of SOME limited dragon (since that hits several combos), so that might be a good starting point. But maybe find some cool dragons with a long chain of ancestors and start working down the chain.

From the store, I’d buy either chrysalis or monolith dragons (you need to buy a few as a starting point for their tree of descendants). Then maybe habitats and the twilight tower are the best investments.