Will Smith.
This is relative considering he isn't as big as other actors, or hasn't been around as long as more established actors (compared to the likes of Harrison Ford; Warren Beatty, et. al).
However, all of Big-Will's movies have done well. From Bad Boys to ID4 to Ali to Hitch. He's never had an actual flop. Hell, "I, Robot" might have sucked as far as a an adaptation goes... But it was actually one of the top box office draws last summer.
---
On a side note, even though Chris said this is a subjective topic... And it is...
The thing with Hollywood is that unless you have bomb after bomb... An actor is still going to get work as long as TPTB think they will bring in ticket sales.
The only major element besides starring in a string of flops that stops an actor from getting work is their age. More than anything else. Hollywood is obsessed with youth and marketing to teens. Hence, agism plays more of a role with who is popular and who is not than actual talent.
Once you hit 30, you are viewed as "over the hill" in the eyes of Hollywood because they figure how is a 15 year old going to relate to someone twice their age? It's erroneous thinking because Hollywood forgets that not only is the actor getting older, but so are his/her fans and the fans' taste in films and what they want to see their favorite actor in will also change with age and maturity as well.
Women, actresses, suffer from this glass cieling more than men, unfortunately. Once a woman hits 25, she is considered "over the hill" and "unattractive" -- Even if she looks like Jordana Brewster -- Because Hollywood doesn't care about longevity. They are looking for the next 16 year old Alexis Bedel they can market to Jim and Johnny 14 year old vs. the 25 year old Jordana Brewster. Again, this is erroneous thinking because the fans of a 25 year old Jordana Brewster are going to be maturing themselves as well and are fare more likely to follow Jordana's career compared to the new engenu's career whom they know nothing about.