This is such a subjective question, so I'll try to be objective. This is the top 10 overall drivers as I see them, in ranked in specific order.
These guys could wheel any type of on road car, stock or mod and place top 5 in every category when they race. These are also the guys that are based in NA, and attend races more regularly.
The shoe ins-
1.Keven Hebert- while he does not have the overall TC wins of other guys, he is the only one that will be top 5 on asphalt or carpet, TC or 12th scale. He ranks this high more because he is either number 1 or 2 in 12th scale in NA and 3-4 in TC.
2.Paul Lemieux- Paul is still the undisputed best TC driver in the USA. He could be higher, but he does not race as much 12th and does not finish as high racing 12th.
3.Josh Cyrul- Josh is having a renaissance late in his now "Amateur" racing career. His asphalt TC results hurt him a bit on the list, but Josh has been the hottest American racer on the rug over the past year.
4.Rick Hohwart- the man, the legend is still getting it done when he attends an event. Probably the second best TC driver in NA.
5.Andrew Hardman- Andrew has come a long way. The kid is raw skill and the wins are starting to tally up.
The next group
5.5 Mike Haynes: leads the "next" group. Can always challenge for a win, in any class.
6.Drew Ellis: Faster than fast, Drew can wheel any type of car and often gets the hottest laps.
7.Donny Lia: when he races, he is a nearly unstoppable force in 12th scale.
8.Cory Parsons: very fast guy that can drive anything on rubber tires. Super fast TC mod guy. Need to stop racing VTA bud
9.Kody Knutdson: current Mod TC champ. Needs to travel more to be ranked higher, but the skill Kody has is very high can drive (and fly) anything.
10.Chris Adams: I race with this guy often and know first hand how fast he is. Chris is super smooth and talented. Top non factory USA guy at world's last year.
There might be other guys that could be argued to be in it, but until they don't get mod results they do not belong as top 10 guys (not that they don't have the skill, that's 2 different things. These guys in the list above would rock the House in spec classes as well- and often do). Modified is harder to drive (not necessarily more competitive or harder to win) and often driving mod translates over to spec classes while spec does not translate over to mod. Spec racing has it's merits, but is also far easier to drive and more equipment dependant.
(*edit- forgot Mike Haynes).