Mono Water is still a pretty great way to go. You get Politoed, Ludicolo, Kabutops, Rotom-Wash, Manaphy, Keldeo, Azumarill, Crawdaunt, and Lord Helix himself. There's a lot of flexibility there, and as long as you can keep Rain up, you have a significant edge against many other mono-type teams.
Flying is great because it has so many secondary type members that it becomes a bit of a hodge-podge team, but it does have the obvious Stealth Rock hindrances on many members.
Mono-Fairy is decent in Monotype, due to lack of common weaknesses and diversity, but there aren't that many good Fairy types around to make a team out of. Mega Gardevoir, Clefable, Azumarill, Klefki, Sylveon, Mega Diancie, Togekiss, maybe Whimsicott. It'd struggle to be a cohesive team I think, though it could work with Klefki providing hazards and paralysis support to lead the match I guess. I don't think this type is the way to go.
Grass I'm unsure about. It doesn't have enough members that cover its weaknesses, especially to Flying.
I'd be more willing to help construct if you had a type picked out specifically. Steel is generally a great choice, Ground can be good, Psychic has a lot of options. Water, as I said before, has an abundance of choices.