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.