For both heat and size... the Jalapeno M and Biker Billy Hybrid are definitely at the top, with similar sizes but the BB being slightly hotter. But seriously, both of them are somehow able to kick my ass in ways that even extra-hot Chinenses aren't able to do, and such a minor heat difference is nothing to me--IMO, they're both very nicely heated, damn hot. A third good variety is the Early Jalapeno, which IMO has the best flavor but is disappointingly small in size. I think I'll try growing it either next year or the year after though, and see if pinching the early buds improves the size of the peppers, because I have a feeling that my plants were somewhat stunted last year for this reason.
I can't say much about the Mucho Nacho Hybrid, other than the fact that it wasn't good enough to remember anything about. But the Mammoth Jalapeno... DO NOT get that variety. I admit that I didn't actually eat any of them, because the plant was overall just so disappointing and it really pissed me off. The Mammoth Jalapeno had practically no corking so it was extremely bland looking, and before the first pepper even ripened and turned completely red, it basically exploded. I walked up to the plant one day, anticipating the first ripening pepper. What I actually saw were ants having a field day running in and out of the pepper, and I immediately pulled it out of the ground. Put it out to the curb and sprayed it with ant insecticide (revenge... hey, sorry, I was pissed...). There were some peppers left to ripen, but if they'll pop before they can ripen, I don't want them.
Mammoth Jalapeno... a horrible, horrible plant. Avoid it for sure.