Jennifer Lopez for me has the most success both in movies and in movies.
J.Lo has this hollywood charm in the silver screen which beyonce, mariah, etc... doesn't have. J.Lo can lead a movie and she's a bankable actress. Beyonce always play a singer in her movies.
J.Lo can act. Her latest film "El Cantante" had great reviews internationally. They said it can be a contender for Oscars 2008 (next next year).
For me j.lo had success both worlds (except for her vocals).