If you want to race singlehanded often then your only options are pretty much A-cat, F16, Hobie 17, hobie 14 and nacra 17.
Other boats like the FX-one are too rare the achieve fleet racing and will therefor always put you in the open class fleet.
It also is strongly dependent on where your sail. Each type is strong in anoher part of the USA.
If you want PLENTY of racing opportunities that are also enjoyable then you should really look at a modern singlehander. H14 and H17 are not able to keep up with modern mixed racing fleets. The others will be derigged and at the bar drinking beer when you cross the finish line. Only the A-cat, F16's and Nacra 17's are capable to stay with the mixed fleets in the way of performance. All these are pretty much formula based. Nacra 17 only in name but none of these designs will remain exactly the same over any long period of time. That is what formula means.
Formula denotes a make-up of class rules that don't intend to make all boats identical but only to make all possible designs equal in performance. So in a formula fleet you can encounter different boats made by different builders that are all the same in overall performance. They still race first-in-wins. Formula classes are totally replacing the old one-design classes of the past. The biggest fleets with the best sailors are currently all formula fleets. This does include the olympic Tornado class as well.
I hope this clearifies things for now.
Formula 16 NED 243 (one-off; homebuild)