Just pick whatever you would be most comfortable with. It isn't very demanding as a healer and you can hide in the chaos as a tank without causing a wipe, seeing as the encounters rarely gives all 6 of them work to do. Prepare for funky queues as DPS, though. I've only done Crystal Tower 11 times (I just roll on whatever Bard piece drops first as well ) and I've found that most groups are actually fairly pleasant, though some may need coaxing to do the fights properly (though even then, wipes are rare in my experience). Only unpleasant one I had so far was a tank and healer duo whining at each other in the alliance chat for being equally bad (he can't keep aggro/he can't keep me alive!)... and that's what, 2 out of 253 potential players? I'll take those odds.