Top 5 what? You need to win some of those fights.
He did beat BJ... but it wasn't nearly Penn at his best, nor was it at Penn's best weight class, and Nick's style was always something that was going to give BJ some trouble.
In fact, Nick's just one of those guys who is going to be a stylistic nightmare for some very high level fighters, and who is going to have holes in his game that make him very beatable for other solid fighters.
If he beats Anderson, it's going to do a lot for his legacy, though. Regardless of where/when/how, he'll be able to say he not only beat Silva, but also outclassed Penn, and KO'd both Lawler, and Frank Shamrock.
Throw Gomi in there (which you should) and he's done some pretty impressive stuff.
Still not the same as being well rounded enough, like GSP and Fedor, or spectacular enough, like Anderson, or a bit of both, like Jon Jones, to beat all comers regardless of what they bring to the cage.