When it is par-building time, the most difficult race type is any race with starter conditions. That includes starter allowance, handicaps, and anything else with such conditions.
The reasoning is that starter races are extremely dependent upon the horses "on the grounds" that manage to qualify in a given season.
That makes for almost no continuity from one year to the next.
HOWEVER...
If you are a player that really follows a given track/circuit to the point where you know (and track) the horses and are able to cite who the competition is in a certain class level (before there is a race even carded) you could really make out in this arena.
This could also apply to NWL claiming conditions and OC.
But it takes work.
The litmus test for whether or not you are that guy is, do you look at a race and ask, "[trainer name] has a horse that should fit in this race. Why isn't he entered?"
Way too much work for me.