All four places are good and are places I frequent. Since I'm only advanced
and not "very advanced" I cannot utilize Chamonix to its full potential,
and it's bad logistics and powder-snow hunting wolfpacks make it not the
best for me. However, it's a real town, so there are things to do at night.

Val d'Isère is a nice place, but it's more intermediate skiing there than
in the other places. Less ugly and more interesting than many other french
ski resorts.

Verbier is a favourite mostly because of the very easily found offpiste.
The 4 valleys look nice on paper, but a lot of it are bad, slow lifts.
Apart from around Verbier the better piste skiing is at the other far end,
which means a lot of transport runs. But the easily reachable offpiste is a
big plus. Nightlife is just plain bad.

S:t Anton is a nice place, and by going by bus for half an hour you can
include some very good runs in Lech/Zürs. The skiing is also good, and last
year I also got some very good powder days and discovered that the offpiste
isn't bad either (I've always had more scarce with powder snow in S:t Anton
earlier). Afterski beats all the other places easily, nightlife isn't as
good as one would expect, though.