Here is The Arena's Viability Ranking of legendaries from OP Rank to F Rank. OP is the most useful choice on your team and F is not recommended. Only legendary 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 ranked at Rank 5, and this list only takes into account the Legendary v. Legendary meta (mythics are not taken into account).
These monsters are too good for the SS+ tier, so they get their own special tier. By far the best legendaries in the entire game. It takes a lot of patience and time (and in most cases, real money) to get them.
These Legendary 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, many of these monsters are frequently used in the Legendary Meta.
Restricted to monsters with mediocre performance in the competitive scene. These aren't the ideal monsters to go for, but are still somewhat usable if stronger monsters are not available.
Monsters with below average performance in the Monster Legends meta, due to bad stats or bland moves. From this point on, monsters won't be viable at all in the Competitive Legendary meta.
These monsters perform extremely poorly in the meta due to multiple major problems with their movesets or stats. These are only viable for beginners and are usually very old monsters.
These monsters are outclassed by a good portion of Epics and even some Rares. These are the absolute worst Legendaries in combat, and generally shouldn't be used in any scenario.