Après-ski: the real point of difference between all three
If terrain narrows the choice between St. Anton and the other two, what happens after 3pm is what actually decides it for a lot of travellers. This is the most subjective page in the whole guide, but the reputations are consistent enough to be useful.
Kitzbühel: social, but not rowdy
Kitzbühel's identity has always been part resort, part destination town — the old town square fills up in the late afternoon with a mix of skiers and non-skiers, and the scene leans toward bars and restaurants rather than the loud slope-side party huts you'll find elsewhere in Austria. It's lively without being the main event of the trip.
St. Anton: the loudest of the three
St. Anton has a genuine reputation as one of the Alps' most intense après-ski towns, built around a handful of famous slope-side bars where the party starts while people are still in ski boots. It's a real part of the resort's identity, not a side note — if that's not what you want from a ski holiday, it's worth knowing going in rather than discovering it on day one.
Innsbruck: a city, not a resort scene
Because Innsbruck is a real city rather than a purpose-built resort village, the evening options aren't "après-ski" in the traditional slope-side sense at all — they're a normal city's restaurants, bars, and old town, with far more variety and far less of a single unifying scene. Good for travellers who want options; less good if a specific ski-town atmosphere is what you're after.