Daggerfall, by FAR. Finished main quest only maybe 3 times, start new characters occasionally even now. Play'throughs' are never the same, I just focus on one kingdom with a character now. Plus the character creation gives you millions of times more options than the later games.
Morrowind I've gone through twice-thrice maybe, haven't checked the mods in years though.
Oblivion through once, uninstalled maybe 2 years ago, missed many new mods too.
Others I've played through once, and ages ago. Not mobile ones.
When I get new comp I'm installing TES3 and 4 again and see the mods that are out by then. But not gonna do it in a year or two. Heck, I might get new rig only when TES5 is out.
Edit: well to be honest, I think I've spent more time in a single dungeon in TES2 than going through a whole guild quest line in TES3 or 4