IMO hitting up the calves with lower AND higher reps is optimal. This can be done in several different ways, going heavy in one workout and ligher in the next, or doing both in the same workout. Heavy range is anywhere from 6-10 reps and lighter anywhere from 15-30 reps.
The soleus should be primarily targeted with higher reps however (seated calf raises), because of the dominant slow-twitch makeup of the muscle. So it should be trained with reps as low as 12 and as high as 30.
Standing Calf Raises- 3 x 8-12
Seated Calf- 2 x 20
Pretty much what I would have said - any exercise is fine (my calves are very stubborn) but I've always found that the following works wonders -
Standing calf raises - use the heaviest weight you can handle - 3 set of 15
Seated calf raises - again, heaviest weight you can hande - 3 sets of 15-20
Toe press on leg press machine - 3 sets of 15-20