MMs are a very strong class in pvp for a number of reasons. Good debuffs, tanky (bodyguard mode makes you effective 3k+ HP), with the ability to dish out huge damage. They're viable in 1v1s, 8v8s and zone pvp as well (not many ATs can do that).
Key powers you're gonna wanna take if you're pvping:
Superspeed/Superjump (standard required pvp mobility powers, without this you won't keep up with anyone)
Some form of -jump (Villain patron power pool grants you webnades which can stack with slow to give you time to debuff/attack)
I'd say most if not all MM primaries are viable, there aren't any that are "bad", it's mostly your secondary that's going to determine how you play and how well you do as a mastermind in pvp. Most secondaries are viable for pvp I'd say.
Viable PVP MM Secondaries: Cold dom, empathy, forcefield (only for forcebolt), kin, nature, pain, poison, radiation, storm (limited viability), thermal, trick arrow
Not Viable: Dark, Sonic, Time Manipulation, Traps
Hope this helps.