Re: Gene Hackman or Kevin Spacey?
Flip a coin.
Both don't play my ideal version of the character of Luthor, I'm not a fan of the tongue in cheek silver age version of any of these characters. Both are fantastic actors.
I guess if I had to pick, I'd go Hackman in S:TM but he had much more to do in that film. With that said, Spacey at least had fleeting moments where he was very vicious and convincing as a contemporary Lex.