Individually? Savage > BM > Zerker > Mercenary on paper
In a group setup? BMs have the most utility with all of their snare styles as well as access to Slam. Mercenaries also have access to Slam but lack the peeling capabilities of BM. Zerkers don't get Slam but make up for it in raw damage output. They're the highest sustained melee dps with a potentially great DPS cooldown that unfortunately tells the enemy team to kill you very quickly. Savage has the highest burst potential and access to the highest Growth rates in the game as well as access to some self buffs that makes them deceptively sturdy.
Mercenaries are probably the weakest on Phoenix due to the lack of melee classes Albion tends to field in RvR. The other three will find a spot much easier.
if u wanna solo/smallman bm>merc, you need engage for archers, svg buffs will drain your hp too quickly without a healer near by zerk just sucks without slam or sitestun, if you only run fg prlly svg they have some nice styles and can do thrust damage as the only mid class except hunter