So the Tux glacier, with a 395cm (13 foot) base and the biggest ski area currently open anywhere, with more than 64km (40 miles) of runs still available, has the biggest and deepest ski slopes at present. It doesn't really close at all, all year, weather permitting. Other options currently open are the Kaunertal and Kitzsteinhorn (above Kaprun) glaciers.
The Pitztal glacier is also open, through to 24th May with its remarkable new Israeli-built plus-temperatures (if needed) snowmaking system. Currently it has a 315cm (10 foot, 6 inch) base at the top so perhaps it's not needed.
The Stubai glacier near Innsbruck is another good choice with 12 lifts and 12 runs still operating and a snow base 5cm (two inches) deeper than Pitztal's! It is open for another month through to mid-June.