Short version: Electronic music in various forms.
Now here's the tl;dr version:
My first fave non-children's artist was Kiss. I wasn't crazy about their music; I was more into the comic-book image of the band. As far as genres go, I was more into disco, the more spacey-sounding, the better (think Donna Summer, Giorgio Moroder, Amii Stewart, M).
After the decline of disco, I picked up on New Wave (B-52's, Devo, A Flock Of Seagulls) and Synthpop (Howard Jones, Depeche Mode, Erasure, Human League), with the occasional foray into 70s Electronic (Kraftwerk, Synergy), Hi-NRG (Dead Or Alive, early Madonna), hip-hop/electro (Newcleus, Midnight Star) and industrial (Skinny Puppy).
Toward the end of the 80s, during my college years, I listened to lots of synthpop/alt-dance (Depeche, Erasure, Pet Shop Boys, New Order, OMD, Information Society, C.C.C.P.). A roommate of a college friend introduced me to more obscure 80s Hi-NRG/Eurobeat like Fun Fun, Modern Rocketry and Divine.
The early 90s saw a turn toward techno (Speedy J, Richie Hawtin, L.A Style), followed by classic Eurodance (Twenty 4 Seven, Real McCoy, Corona, Cappella).
In 1996 a local DJ-friendly record shop demoed Paul Oakenfold's first Tranceport compilation and BT's "Ima" album for me, and I became a big Trance fan. For me, trance was a refreshing update of the electronic instrumentals I already liked, to an even more dancefloor-friendly beat.
A few years later, I discovered a burgeoning Modern Synthpop scene, with bands that were carrying the torch for the genre that was largely abandoned when grunge overtook the US music charts. I downloaded free tracks from Provision, The Echoing Green and Cosmicity on mp3.com, among others.
Most of the time since then, I've been bolstering my collection with "back catalog" releases of some of these bands, with a Top 40 dance-pop track or two on rare occasion (Lady Gaga, David Guetta) and a couple of new Modern Synthpop artists (Moonlight Cove, Rename, The Dignity Of Labour).
Unlike many people I know, I've never really stopped listening to any of these genres; they're all represented rather well on my iPod. If I had to pick my fave genres, I'd have to put Synthpop (80s and Modern) as #1, followed somewhat closely by Trance. Right behind is a three-way tie: Eurodance, Hi-NRG and New Wave.
There are genres I'm not a big fan of (country, hard rock/metal), but I don't particularly "hate" any genre. I guess that's the hobbyist musician in me; I can appreciate musical talent in just about any genre.