In reply to craig1983:
Best in what respect? Wicking, warmth, comfort.
I use power stretch for warmth but prefer polyester for wicking and general all round use. Both are reasonably comfortable. I've never really rated Merino, it seems to fit somewhere between the two but takes an age to dry when washed.
I also find that tops made by Nike, Puma, Adidas etc. work just as well and cost half as much as those with mountaineering manufacturer logos.