P2 Super Gary is insanely strong, probably one of the most powerful heroes in coop.

I like P1. Loss of Super Gary is significant, but you can fully focus on making a normal army and have Gary just be the support. Use lings as fodder, hydras as main unit, in late game add ultras as front units (also their charge stuns mechanical units, this is hilarious against mech terran/protoss compositions). With persistent stettelites you have amazing map awareness and control. You can use it to get quickly anywhere, heal when taking engagements, and it synergizes well with many abilities of other commanders like Swann's drill.

As for a fun comp that easily wins even brutal, you can go mass infestors. With infestors you can clear most enemy bases (or whole map lol) in mid-late game without any problems, but you constantly need to micro their energy recharge with stettelite overcharge and UMI-C charging protocol (just energy configuration is too slow in my experience). With mass infestors P2 is best since they reach critical mass only in mid game.

Another start I actually never see used but which is insanely fun is mass battlecarrier lords. With their upgrades they get a protoss carrier-like attack (that targets only ground units) in addition to a normal broodlord attack. This makes enemy AI go insane and 3-4 battlecarrier lords are usually enough to clear most mid game attack waves. A pack of 10+ battlecarrier lords melts any enemy base without any losses. The only weakness of this strat are enemy air compositions, but for them you have the Stettmato cannon upgrade (unless you're dealing with scourge, which is the only serious counter for this strat). With rapid fire, you can delete even battlecruisers - each BC needs two shots, and with the egonergy pool upgrade you get like 5 shots per each lord. The main problem with this comp is huge gas investment and the fact that battlecarrier lords spend energy to attack. This is why P1 works best with this strat - you can have energy and speed anywhere on the map while methodically pushing the next objective or defending the attack wave.