The Frankie Goes to Hollywood 12"s were all mostly excellent. I made a cassette with all the mixes of Relax on it, no gaps and hence... a 30 minute mix.
Other overplayed 12"s from my youth are Tears For Fears, Mother's Talk: Beat of the Drum Mix. Thompson Twins, You Take Me Up Extended Mix. I'll not mention New Order's Blue Monday, but i played it a lot. Duran Duran, The Reflex, Nile Rogers remix, Tears for Fears, Broken/Head Over Heals/Broken (I think it was a 10"). Phil Oakley & Georgio Moroder, Together in Electric Dreams and several more which i can't currently recall.
In my early 20s i got into funk and disco... a couple of stand-outs being Parliament's Red Light being a superb bass heavy throbfest, and the 12" of Donna Summer's version of Je T'aime is silky and sumptuous throughout... and strangely has no B side. It's just glossy grooveless plastic. Never seen that before, or since.