It probably depends on what equip you run and what level you are use to racing.
I love the F100 series. We race every track in the area (grange, calspeed, parris, Adams and willow springs). Best of all the guys are super nice to hang out with and very helpful. There are 3 levels, so if you are experienced, you will race against some of the fastest people around, if you are a beginner, the class still has 12-15 people in it every week.
Check out f100 karters online.
Some people will race the F100 series along with other series as well. The F100 series also has a shifter class as well (obviously not the same 100cc motor, but some guys like to run shifters and 100cc on the same day).