Agan; any advice you can provide me on products you use, or know of, would be of great help. The products I mentioned before (outside of the pixel studios ones) are bolt-ons that I could see myself using, it's just weather they are the best product.
CoreMelt: Lock & Load X, mTitle 3D, Twixtor, FxFactory Pro (less so) and Primatte Keyer.
CoreMelt: tested L&L, nice, but using FCPX' own stabilizer, happy with it, using TrackX (rarely), a bit strange interface, from an enthusiastic Motion users perspective, Motion covers 80% of TrackX 😉
… generally: 'trackers' are nice for some fancy titles ('attached' to objects), replacing content and fixing flaws … but I use'm less & less.
(aside, I expect some 'smart' tracker in some future gen of FCPX, highSierra would allow that on an OS-level… <tea leaf mode off>
mTitle - no hands on, if I need custom ones, I build my own in Motion. Published ~100 free ones @fcpco.
Twixtor - done lots of football reports, season-in-review stuff = slowmo is a Must! … had a look at Twixtor: a) no trial avail - too bad!, b) basic functions are covered by FCPX' Optical Flow, c) never understood Twixtors interface to define background/foregrounds … when you record, with OF in mind, little need to spend so much money - imho!
FxFactory - what Luís says …
Primatte Keyer - no hands on, but claiming myself, using World Worst Greenscreen in Movie History (ever, not to say eva!!!😝) in some zero-budget school projects, I'm a huge fan of FCPX' keyer. Follow advice from rippletraining and it gives awesome results. Little need for 3rd party products. But if you do, have a look at Simon Ubsdell/Tokyo Production … Simon IS one of the best, undoubtedly …
FCPX comes with an arsenal of 'effects'; many miss the options, the 'knobs' to play with.
Asking for 'best' provokes instant reply 'define best' … do you need a tracker, replace content, use color-key? Discussion gets soon academic: What's best for me is of no use for others. And vice versa.
No word yet about topics much higher on list: color correction, grading, audio (!!!) …
Final word: don't fix it in the post, be practical.