For myself, it is True20 (if it counts) and 3e. However, 3e would require some house rules, some third party stuff, and very little in terms of WOTC supplements outside of Unearthed Arcana (ok supplements include: MM2, Fiend Folio, the dedicated monster books, BoVD (DM only), the environment books, Heroes of Horror, Heroes of Battle, and a few select elements from various other books).
As for other d20 fantasy variants that I would consider giving a try, they include Grim Tales, Swashbuckling Adventures, and Rokugan.
Grim Tales, because I like the d20Modern setup. The question would be how easy it would be to bring in Elements of Magic:Mythic Earth and Green Ronin's Psychic's Handbook.
Pre 3e DND, 4e and Pathfinder, I have no interest in.
Pre- 3e DND: Despite loving certain setting like Darksun, Ravenloft, Al Quadim, 1e Forgotten Realms, and the Known World Gazateers- I stopped playing and running editions prior to 3e for a reason.
4e: I will not play 4e. This may or may not change depending upon third party support and the products they produce. If third party's provide alternative mechanics on the level that EN Publishings Elements of Magic Revised (or Mythic Earth) and Artificer's Handbook (Mystic Eye Games) did for 3e, I'll give 4e another look. Currently, there are a few changes that I do like, but outweighed by the number of things that I do not like. However, I am hoping that someone will rewrite the 3e player races to be more like 4e with non-biological abilities being feats.
Pathfinder: I don't like most of the changes in the alpha or beta. However, I might borrow how they handled spells costing XP as well as a few of the spells