Here is The Arena's Viability Ranking of Original Mythics from OP Rank to F Rank. OP is the most useful choice on your team and F is not recommended. Only Mythic monsters are covered in this ranking.
Disclaimer This list is based around monster's performance in The Arena (PvP), NOT Team Wars, Duels, etc. The monsters are also ranked at Rank 5.
These monsters are too good for the SS tier, so they get their own special tier. These are some of the top regular mythic monsters in the game and are highly recommended for your team.
Restricted to the best of the best. These Mythic monsters are very good but cut just short of OP rank for having either lower stats or just weaker moves compared to OP rank. Despite that, they're easily some of the best in the game, and a team should really consider having any of them.
Restricted to monsters that perform great or at least good in the competitive scene. The ideal monsters you should tend to go for if you aren't able to obtain higher-rank monsters.
Restricted to monsters with average performance in the Monster Legends meta, due to average stats or situational moves. But they still are considered as viable options for your team.