The most popular Pokemon, up to Top 7 in National Dex Monotype, are all water-types.
+ ---- + ------------------ + --------- + ------ + ------- + ------ + ------- +
| Rank | Pokemon | Usage % | Raw | % | Real | % |
+ ---- + ------------------ + --------- + ------ + ------- + ------ + ------- +
| 1 | Toxapex | 22.81245% | 2393 | 6.768% | 1887 | 7.231% |
| 2 | Swampert-Mega | 20.13984% | 2496 | 7.059% | 1915 | 7.338% |
| 3 | Pelipper | 19.70808% | 2450 | 6.929% | 2187 | 8.380% |
| 4 | Greninja | 19.43604% | 3049 | 8.623% | 2185 | 8.373% |
| 5 | Slowbro | 16.85180% | 944 | 2.670% | 676 | 2.590% |
| 6 | Dracovish | 15.18367% | 1619 | 4.579% | 1196 | 4.583% |
| 7 | Tapu Fini | 14.39100% | 1030 | 2.913% | 751 | 2.878% |
This confirms that Water-monotype, especially rain teams dominate Nat Dex Monotype. As such, you can choose to use your own mono-rain team, or try counters against them. Grass and Electric mono-types are best against them.
I also tried asking maroon, who is the tier leader of Nat Dex Monotype. Here's what he told:
Maroon: Water, Psychic, Flying, Steel and Dragon are all decent starting types. You asked me about anti-water types, which grass is fantastic at. So I wanna give you advice for building grass monotype. Mega Venusaur + Ferrothorn is all you need to form a defensive core. Don't do Cradily, since it makes Grass-monotype more passive. Next, you need a Rillaboom, Swords Dance or Choice band, both work. Then find your other non passive Pokemon, and make your team.
So, Water, Psychic, Flying, Steel and Dragon are your best bets. If you want to go anti-water since Water is most common, a defensive core of Mega Venusaur and Ferrothorn along with Swords Dance/Choice Band Rillaboom is your way to start.
Before making and choosing your monotype, read this post. It's about the viability ranking of each type.
You can also see about sample teams here for each type, and make your own team similar to samples.
Well, that's all. Hope this helps!