Listen I do a lot of telecine/digital intermediate on a very expensive sytem, and I am very impressed by the color corection tools in FCP, if you know what you are doing you don't really need more, unless you go out to film, in which case you should do it with a professional colorist.
I you are not comfortable with the "science" of colors, get something like magic bullet which gives you "looks" that you can tweek. Also if you need to do serious color corrwctions you will need a calibrated professional monitor.
If you are doing simple stuff, the FCP tools are fine. Especially if you don't know any better.
But if you are serious then there are tools for you.
Color Finesse is a plugin for FCP that gives you editable spline curves on R, G and B plus multiple instances of secondary correction.
It also gives you much better waveform monitor than the stock waveform.
If you want serious correction the hockey pucks of FCP don't stand up to direct manipulation of color curves, gamma curves, etc.
There are reasons all of the other color correctors allow you to directly edit the curves.
DaVinci, Avid Nitris, Avid Symphony, the entire Discreet line, even Premier Pro 2 and Avid Xpress Pro give you access to curve editing.
Maybe it's just my photoshop background but I feel even high end color systems lack full creative power. That said, I've been very impressed by Nucoda's Film Master system which is the closest thing I've yet seen to an ideal setup.
As for the best color correction for FPC - that would without doubt be Color Finesse by Synthetic Aperture.
I wonder if he's asking which FCP CC is best --- the "three-way" or the regular CC?
I agree with Shaw -- if you're coming from a Photoshop background, everything in FCP seems woefully inadaquate. I'll be checkin out that "Film Master" system.
P.S.: I like the "three-way" for more intensive CC, but often find that the regular CC will do the job.