Just got my Fee Bros. bitters set
The orange bitters work well in a Manhattan, or classic Martini. I also like a dash in in a Negroni.
The peach bitters also lend an interesting background to a Martini, though I've liked them best in a recipe that I think is on the bottle -- sherry with dry vermouth. A nice apperitif.
The aged bitters work well in a Manhattan.
I've also used the lemon bitters in a martini for a bit of variety.
The only one I haven't cared for is the mint bitters -- it just hasn't blended well in the various drinks I've made with it.
And don't forget, a dash of bitters of any flavor can really pep up an ordinary glass of sparkling water.