My thoughts (early warning: I tend to be long-winded...)
I've stayed at all three. Stayed at Hard Rock the least recently since they had a change of management that seems to have interfered with whatever our standard booking situation was (I don't do the booking for our family, I don't know details).
I'm pretty sure we usually get standard rooms, though we did get a suite at the Portifino once, but the suite was basically just two rooms right next to eachother with the connecting door open, which was kindof a letdown.
I have to say, I've always found the actual rooms in all three hotels more or less identical, with minor differences in themeing. I don't prefer the rooms at any one hotel more than the other.
As far as theming, I think I prefer Hard Rock, but then I'm a male in my 20s. I enjoy the rock memorabilia and hollywood glitz it has. Royal Pacific is also nice. The Polynesian feel, as others have said, gives it a somewhat soothing and exotic flavor. Portifino is my least favorite of the three. I imagine it would be interesting to someone who REALLY like Italy, but to me it's just kindof... no big whoop. My sister and mother seem to love it though, so to each his or her own, I guess. My other biggest problem with Portifino however, which brings me to my next point, isss...
Location! Portifino's sucks. Don't try to walk to or from the parks if you're staying at Portifino, it doesn't look far when you're on the boat, but boy howdy it is. Royal Pacific, by contrast, is a fairly walkable distance. I still prefer the boat, but my family will hoof it from time to time. Hard Rock wins in this debate, it has a great park adjacent location, less than 5 minutes away. You can practically step out of your room and be in the park.
So all and all, I'd say Hard Rock or Royal Pacific. But I know some members of my family would disagree. At teh end of the day I'd say do what we did, try them all out alternately on multiple return trips until you settle into one. (We seem to have settled into Royal Pacific, for the record. For a combination of the reasons already mentioned, and the whole "It's Cheaper" factor, lol.)
So all and all