I don't know about most viable, but here are the Top 10 most used Mons at the 1760 AG Benchmark. Basically status abuse and BP/Moody, along with the strongest offensive threats in Ubers.
1 | Rayquaza | 66.54640%
2 | Klefki | 63.63702%
3 | Arceus | 56.78001%
4 | Groudon | 40.79504%
5 |...