Leading Angband variant? I'd pick two, Oangband and Sil, for very different reasons.
Oangband hasn't been developed in years but who cares, it's "done". It's tough, balanced, stylish, no-nonsense game. For me, it's Angband done right. It's the leading variant because it has already reached all its goals. Most of the rapidly developing variants are still a long way from reaching their own goals. (FAAngband is based on Oangband and is actively developed.)
Everybody loves Sil. It's a totally different game from Angband. It's a young game, but already mature and well-balanced. I haven't actually played it yet but I know that when I do, I'll be hooked for months.
My own variant, Halls of Mist, seems to develop more by revolution than evolution. Each new version is an experiment of sorts. I like testing wild ideas by playing. Maybe by 2.0 I've reached something that I'm actually happy with, and can continue with evolution. It feels weird when people compare my little hobby to other Angband variants. I actually like Larn more than Angband, and it may show in my game.
Arthandas, Hengband and Entroband have persistent dungeon of sorts. The dungeon levels are remembered until you return to town, and then the memory is erased.
