Honestly i think DPS is the hardest.

you can get by as a crapy tank as long as you have the gear, you can get by as a crappy healer as long as you at least heal.

But as a crappy dps you will get fired ASAP from any raid.

in ICC you can get by with both average healers and tanks, but if your DPS suck then all encounters will become a pita.

Tho my experience is limited as to having played tank, healer and dps in ICC 10