TomTom is the best, its maps are perfect for Europe.
Though, for USA I'd prefer Garmin maps (much more side roads, especially in rural areas, and updating them more often then TomTom, I believe). So, the choice depends on where you are.

And I agree with NightHiker, Garmin is much more versatile. TomTom has in-car-only solutions (excluding their PDA software product).