Hmm...given how I had a lot of fun reading Catwoman Tales, I would say Batman + Catwoman would make the most fun crossover.
And Terry Leatherwood gave us both Supergirl (well, Kara) and Wonder Woman.
Oh, you're talking about the show, not fanfic. Hmm... I think for Lois and Clark the show, Catwoman would have made the most fun as a catburgling foil for Lois's investigative skills and Superman's attempts to catch her. She could have stolen Lex's green diamond. He insists it's not an emerald.
Michael
