All of them are annoying. -_-" Mostly depends on how you use and build them though since none is better than the other. I can give some things I've come to know about these classes in the pvp world haha
Sharpshooters are good with traps, they love to set down stuns, ground DoT, exploding bombs thats fixed like 50% HP? Or something -_- and what not. Good for teaming up with melee classes or magicians who can stun lock someone. Damage wise theyre not that high, they don't fair well against magic users since they are squishy -- but if partnered with any class with stun they are a good asset. Theyre not single PvP oriented, they work best with a team.
Adventurers seem good on 1 on 1 pvp, they have a reflect skill that can piss off anybody. I think they have pretty good cooldowns...Im not sure much on their pvp usage as I never had a lot of Adv/WW in my raids after third jobs =)) But what I know is that its fairly challenging to build them and might need to invest -- they are squishy, but they do have shields and a heal skill as well >.o" I think all mail users do Dx
Shadow Walkers are good for debuffing and can handle themselves well on their own. They have heals, shields and a stupid debuff that takes away 20k of All Def, debuffs that take away evasion, hit chance...everything. Honestly this class is too OP for words. High evasion rate, fast cooldowns, high attack speed and good survivability. It IS also highly dependent on what you are wearing -- if you are not build well gear wise, its going to be bad. A very built SW can be a real pain in the ass as solo, and especially partnered with high damagers like a Sorc or a Vanq. So far this one has the most props as PvP.
Another is the Sniper. This class is geared towards magic users with countless silences as well as being long ranged themselves. Good for ninja killing or helping others through pushes, immo, and other debuffs its the ranged equivalent of an SW. They're better partnered up with a melee though. Solo wise it's okay, it has advantage over casters due to silences. Squishy against melee, they might be bad if you have mail users as enemies.
Overall, it depends if you're a solo-person or a team player. Solo you can probably choose SW better to build. If team player, Sharpshooter might be a good option. Adventurers are a bit of an in between.
Might wanna ask an actual mail user though xD! Im looking at this through my own experience with fighting them~