I'm sure that the directors brought out the best of her, but you could say the same about the male actors, and you can't say that Lopez didn't more than hold her own in Out of Sight. The scene where the thug tries to assault her in his house is superb, ratcheting up the tension and then having her bring down the hammer. And her chemistry with Clooney was sublime.
She's done quite well for a musician turned actress, better than pretty much anybody else, surely?
Aside from the ones above - Out of Sight, U-Turn and The Cell - I thought she was great opposite Redford in An Unfinished Life, and had some nice early turns in the likes of Money Train (Snipes and Harrelson), Blood & Wine (Caine and Nicholson) and even the first trashy Anaconda (although Voight's hamtastic villain stole that show).
As noted, I think she had to balance her pop life / american idol gig etc. with her film 'career' where nobody really took her very seriously partly because she was a huge sex symbol, and I also think the Affleck years didn't help (Gigli), nor multiple very public marriages / woes.