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its up to you really, what are you intended goals? And how many tanks/what size tanks are you working with? I can help you narrow it down a little, but in the end you should still pick a species that you really like, because that's the main reward for breeding.
I have loads of BN pleco's available, $12 + shipping for a breeding pair of browns that also produce albino and calico fry. BN are a good start for learning to breed other types of pleco's.