Warzone Assault I believe is the fastest place to collect RP, potential for very short matches and bigger payout that arena typically. I feel I should say though be very sure you want that Reach pack with your 120k, from a straight up unlocks to RP ratio burning it on silvers (if you haven't completed them yet) or golds (if you have) gives you vastly more than that reach pack will. And you'll still have a shot at unlocking whatever it is you particularly want. And the reach pack is just as random, so it may have only one piece of the armour set you're hoping for, or zero pieces. Not meaning to rain on your parade at all, just wanted to give a little warning in case you want to change your mind!
Yeah, I know I could be unlocking other things, but I don't play too often anyways (just 'cause of other games to play), so I don't really mind spending all my time trying to accomplish this one time. I've made 92,000 RP in 2 days, so if I really wanna unlock everything else, I'll do it later.