Aside from Weaponlord. My pick is probably MVC2. I tried to learn it, but was totally overwhelmed. I respect that community don’t get me wrong. But the amount of shit you have to know; practical chain combos, infinites, when to call an assist, etc. Is really hard. I’m going to try to get into MVC3 though (so long as the netcode is really good).
Honorable mention to 3S which does take a long while to get decent at. And Virtua Fighter (the whole franchise).