G1
G2 or
HS1?

Viability on disc brake road bike?

I've got a G2 on the front of my bike (with BB7 road caliper). It does OK and looks nice enough, but it squeals like mad after a fresh rain. I suspect that most (if not all) rotors would. If the price isn't too much more, I'd probably try the HS1 just to see if it's any better.

More info please, mechanical, hydraulic, pad & caliper, flat roads or climbing?

For braking hard at speed and/or downhill, get the largest diameter, beefiest rotor that will fit; the HS1 is not beefy.

I have the g2's with avid bb7's on my crux (cyclocross bike) and they work great, I think they are 140 and 160mm rotors. However from what I have read it might be smart to double check that the rotor you are considering is compatible/recommended for use with your brake model as such things can lead to catastrophic failure and injury.