Competitiveness is tough to generalise since it varies by region, but I'd break it out from easiest to toughest like this:
Lightweight Supersport (stock inlet size on air filter)
Formula-40 (if 40+ years old, top 3 in MW are fast)
Lightweight Superbike
Lightweight Grand Prix
Thunderbike
GT Lights (30-min race)
SuperTwins
...and you can race up a class if you really want to.