Being sexy is a little bit complicated thing, physical attraction is not always enough for it. It needs a good mixture of intelligent, character, authenticity, mystery, humor, something animalistic, raw.. so on.. Probably everyone has his or her own definition.
TOS: Captain Kirk, Spock (my first crush), Khan and all the beautiful ladies
ENT: T'Pol, Trip, Shran, Reed, Hoshi
TNG: Ro Laren, Kurn (I never understand the hype for Troi and Picard. They are for me as sexy as a food fungus, at least Picard is a great character played by a magnificent actor, but sexy? No! Maybe he was very old for my taste, as TNG aired. Riker is so easy to have, that kills any kind of attraction imho.)
DS9: Jadzia, Weyoun, General Martok, Keevan, Elim Garak
VOY: Seven of Nine (not always)