A is for Altair, a major star system that may or may not have been in the Federation during the time of TOS
B is for Bolius. The musicians there really know how to sing the blues.
C is for Cardassians. They have spoons on their foreheads.
D is for Deneb IV. James Kirk and Gary Mitchell visited it more than a century before Jean-Luc Picard did.
E is for Ekos and it's Nazi copycats.
F is for the Federation Science Council.
G is for Gowron Chancellor of the Klingon Empire whom Worf killed in hand to hand combat in the DS9 season 7 episode Tacking Into The Wind
H is for Hengist, the human chief administrator of Argelius II in 2267 and the last known host of the Redjac entity.