Endlers, Gambusias, and Mollies. Mind you, it's a bad idea to hybridize Endlers and Guppies, and Molly/Guppy hybrids are near impossible to make. Gambusia/Guppy can sometimes work, but Gambusias can be very aggressive, so it's a risky cross.
Also, you know all those fancy-tailed guppy varieties (such as the double swords) with the bold blocks of color? They are mainly endler's hybrids anyway - so no need to hybridise guppies with pure endler's - you can already buy fish with endler's in their anscestry (and you may even own some).