Magic and Range are both extremely fast if money is not a problem.
For the fastest Magic XP, you would Barrage Skeleton Monkeys at Ape Atoll in Maxed Mage gear. You would do the same thing for the fastest Range XP, except in Maxed Range gear with Red Chins.
That is the best XP rate per hour you will get out of a combat skill.
EDIT:
I should point out that with Quatakai's suggestion, Rock Lobsters are fast but slower than Skeleton Monkeys (except Rock Lobsters actually drop a lot of charms, which is useful in other ways). If you care ONLY about getting Magic up, I'd go to Ape Atoll (but if you want to train Summoning down the track, hit up Rock Lobsters)