You could mount a case for any of them.
Clarke - a triple and a double. Superb captaincy throughout. That should pretty much seal the award, but then again, he did fail with the bat in two of the tests, and for a series award, that may count against him.
Ponting - consistent. Only failed in one innings. 2 50's, a hundred, and a double all in pressure situations. Remember the 1st test in Melbourne when his partnership with Hussey was so important (Australia were 4-27 when Clarke got out) ? His 100 was important in building the partnership with Clarke in Sydney, whilst his 200 was equal (albeit slower) than that of Clarke's.
Hilfenhaus - Consistently outstanding. Just look at the numbers. Not only that, he was able to bowl more overs than the other bowlers which meant not having Watson or Lyon (who didn't bowl lengthy spells) didn't weaken us as much.
Siddle - Again, outstanding and consistent. I thought he gave us the breakthroughs when partnerships seemed established. Also contributed some runs (esp. Melbourne, but also Perth when he was 3rd highest), when it was important.
In the end, I would go Hilfenhaus. The ability to bowl long and terrific spells, as well as being brilliant in ALL 4 test matches is important. Consistently is the key. VERY hard to play VERY well in ALL the innings.
Clarke wasn't (batting-wise) in 2 tests, whilst Ponting doesn't have the volume of runs that Clarke made (nor was he as convincing). That being said, it was more of a bowler's series except for a few partnerships, so that may carry some weight.
I think Clarke will win it though - his captaincy has been brilliant, but moreso because of his historic triple and then the double.