"Best" as in "most reliable" is always the latest OFFICIAL navcore from TomTom, which is 7.903 for the TomTom ONE v2.
If however you would like to use a map with IQ routes and ALG, then most likely you will need a v9 navcore. My first choice is 9.170 SE, which you can download from the following link. You can also try 9.061 from the same thread:
Navcores 9.061 & 9.170 for 32Mb ONE models [by mark cat]