Attention! The tier list comes in effect 31st of October (6D patch/rework)!
The following is a compilation of how well a mercenary fares into the current PvP (arena) meta. There isn't a PvE meta, as all units can be used there freely, but generally, if a unit performs well in PvP, it will also in PvE. Keep in mind that these tier lists are everchanging, especially when new units are introduced and shake the meta.
To further read the justification about this tierlist, see Baloo's reddit thread : https://www.reddit.com/r/brown_dust/comments/deuk7i/pvp_regular_arena_tierlist_post_6d_update/