It depends largely on how big of an issue portability, comfort and sound are. I've tried out all three of these and I own the SR60s. I can only say I disagree totally with Sanka. I've listened through my Grados for 6 hours straight and wasn't the slightest bit "fatigued". Once they're burned in (mine are just starting to get fully burned in at about 150 hours of play time) they're more clear than they are bright. I've used them on my rockman portable amp and the sound is much much clearer than what I was used to hearing out of it, and it actually helps, since I notice when my playing is getting sloppy better.
I have only tried portapros and I briefly had the sennheisers, but I returned them when the left driver developed an annoying buzz on the second day. Both sounded good, but weren't nearly in the ballpark with the grados.
However, both were leaps and bounds more portable than the Grados, both leaked out sound less and both let less sound in.
I'd also say that the Grados are the most comfortable out of these, because they don't make tradeoffs for portability. Once the flat pads "break-in" and conform to your ears a little, they really are quite a joy to wear, even for long sessions. The portapros didn't seem bad, per se, but I really couldn't see the metal band being as comfortable as even the vinyl band on the SR60s. Just realized you were talking about the PMX 100, which is a slightly lower version of the PX 100. I didn't have real comfort issues with the PX 100, but I did, at times feel like it was going to fall off my head. The PMX 100, from what I can tell, adds earclips into the equation and subs in a less comfortable headband. I've never tried it, but that thing looks like a nightmare comfortwise. And I would guess, best case scenario, it might sound as good as the PX 100, but looks like a slightly lower quality version.
Hope this helps.