What's wrong with the navcore you're currently using? Navcores don't always need to be updated if they are running smoothly. If it works well, just update your map and activate it.
1) "latest" navcore, techinically speaking, is the latest official navcore for your device that TomTom Home gives you, which is probably version 8.010 for the ONE XL.
2) "fastest" navcore is usually the official navcore. Another one I find as being the fastest on ONE models is 8.562 SE.
3) "best" navcore for me is the one that works the fastest and smoothest with no lag and maximum RAM available, hence 8.562 SE.
Best navcore for someone else might mean the latest with a new, different looking menu, maybe with a few more functions. But if the unit is going to lag while I'm on the road and take longer to respond than usual, then I wouldn't care about all the fancy looking menus and functions that I wouldn't use anyway.
So you really have to try a whole lot of navcores, as I have done, and find the one that suits YOUR needs. Happy experimenting.