I have three competitive mono teams right now: Fighting, Ghost, and Flying, all of which work to varying degrees of success dependent on how much of an idiot my opponent is(meaning my Flying team wins WAY MORE THAN IT SHOULD).
My Fighting team runs Infernape, Keldeo, Lucario, Conkeldurr, Mienshao, and Breloom, and has literally not changed since Gen V. Having three things that can work as a lead gives me plenty of choosing power for my opening move. The coverage I can get is actually really high, and I'm not exactly screwed over when fighting a mono Ghost or Flying team.
The Ghost team is about what you would expect: stall, stall, and stall some more, then murder with powerful sweepers should that fail. With Gengar, Trevenant, Sableye, Golurk, Aegislash, and Cofagrigus, I have more than enough ammunition with which to cause many a ragequit. The core of the team is Sableye and Trevenant, being able to stall out many a thing through a lovely combination of Toxic/Burn and Leech Seed backed up by Leftovers/Recover and Leftovers/Harvest/Substitute, respectively. If you haven't seen a Cofagrigus sweep, let me tell you that is a horrifying sight.
Finally, the Flying team. The weakness to Ice is offset only by Skarmory and Gyarados, but that doesn't seem to keep me from steamrolling through mono Ice teams who have no idea how to use Ice Shard. Skarm is joined by the lovely ensemble cast of Landorus-T, Yanmega, Swellow, Gyarados, and Vivillon(who will completely annihilate you provided you don't OHKO it, which surprisingly hasn't happened yet). Yanmega is a reliable enough lead, but Vivillon can do it, too. Do not underestimate this team.