Since no one else has suggested it I'm going to say plant. No one can lock down a spawn as early, often, and effectively as plant. Seeds of confusion is available at level 8 with a huge cone and a 1 minute recharge that effectively neuters a mob. Then later you get a usual AoE hold and then carrion creepers that allow you to lock down twice more. All three can safely take an alpha strike for a team (although some minor stealth would be suggested). Carrion creepers are also where you get some nice damage (and AoE damage even). Also of note, Plant for whatever reason has its AoE immobilize power deal twice as much damage as most other control sets. Last, the pet is decent.
Other sets can start to catch up to plant's control later, but early on it saves those lowbie teams from themselves.
Honestly any secondary can work. I tend to go rad for the extra damage/recharge/-regen/-defense/-tohit/+recovery/status resistance. You know, all the awesome things rad offers.
The funny thing is I got both a plant/rad controller and a plant/ice dom to 50 during live and ended up not even noticing much killing speed difference between them. Plant does so well on its own.
Edit: Forgot, if you go rad you also get EM Pulse which is yet another AoE hold you can use to shut down the mobs.