I like Fairies, but you may want to cheat and include fairy egg group. There aren't too many Fairies yet.
I made a Mono-Normal team that I really like. It's only glaring weakness is stab special fighting moves, but thankfully there aren't many special fighting types. (Crosses fingers for a decent Normal/Ghost next gen)
I did get beat by a mono-grass triples team a few weeks ago. There was a Breloom (was one shot), Foul Play Shiftry (my team was built around debuffing, which Foul Play ignores), and a Whimsicott.
I don't remember what the rest of his team was, we got down to 1v2 but my Miltank couldn't get any leverage on his Whimsicott and Shiftry. It really makes me want to make a Mono-Grass though... and pack some more fire moves on my team.
Mono Grass is awesome in doubles/triples imo! I was mostly inspired when I discovered rototiller.
She walks like a horse God http://i.imgur.com/rk6qd7r.gif