It is all about the pads. I bet the most comfortable would be the MS-1 or SR-60, because of the comfy pads. Though if you put these pads on the SR-80/125/225, HF-1, RS-1/2 it would be just a comfy, because they all weigh next to nothing. Featherweights my man. So easy on the head, you almost feel like you are not wearing headphones.
Now the SR-325/325i, MS-2/2i, and the HP-1000 series might not be as comfy from the added weight of the aluminum housings. Still I bet they don't weigh much.
IMO the bowls are very comfortable (I just can't take the sound). The flats are ok comfort wise, but sound awesome. The HD414 pad is about as comfortable as the bowls, but sound in between the bowls and the flats.
Just in case you were not aware, with Grado's it is all about the pads. They can make the headphone sound and feel completely different.