Let's see.
Uru can take control of an opponent's monster when you tribute on of your own. That could be really useful, for you can tribute a weak monster for a powerful card like Judgment Dragon or Heraklinos.
Aslla Piscu's destruction effect is nice, and it can do some decent burning. However, its ATK is kinda low for its level at 2500.
I don't think Cusillu's effect is very useful, and a 2800 ATK isn't much to brag about for an immortal, so he's out of the picture.
Ccarayhua is about the same as Aslla Piscu, except with 300 more ATK.
Ccapac Apu can do a lot of damage, and its 3000 ATK makes it even easier to deal that damage.
Chacu Challhua's effect isn't very good; you're better off attacking with it.
Wiraqocha Rasca can easily be 3100 ATK, the highest of the immortals, but the effect is situational. You probably woudn't have 3 other cards out when you summon an immortal.
I'd say the best one is Uru. But in the TCG right now, I'd say it's Ccapac Apu. Hope this helps!
Edit: What you people don't get about Chacu Challhua is that he has to be in defense position. Why change it to def for only 1200 damage to stop your opponent's battle phase when he can't be selected as an attack target anyways?
With Immortals, you wanna deal as much damage as possible before your Geartown gets destroyed and you have to resort to Gadjiltron Dragon. That's why Ccpac Apu is the best one right now.