Uhmm that's a hard question. I mean I can't just say that 'this' is a better class compared to another. Tell you what, I have some FD chars here and I can just tell you the pros and cons of each class that I deem worthy for PvP.
Arcanist - check out this guide that I made. This pretty much sums up all that I know about Arcanist.
https://mazeyflyff.forumotion.net/t7020-a-comprehensive-guide-for-arcanistsMentalist -
https://mazeyflyff.forumotion.net/t7068-a-comprehensive-guide-for-mentalistsForceMaster - If I wasn't a big fan of Arcanist, I'd go with this one. I hope you know what a "switcher" means. Well, it basically means that you have a damage gear and a hp gear. With that being said, a ForceMaster with HP gear is almost unbeatable (unless faced against an experienced Harlequin). I said that because the max HP or a STA geared FM is around 720m. While the max HoP of a FD Harle is around 690m - 710m if I remembered correctly.
If you don't have enough RPs for a switching gears, you can try hybrid FM. I did this before. I have like 400m HP with around 710m Asal and that is without seraph's buff.
Harlequin - This class probably has the potential to be a better class compared to FM if used correctly. I mean this class has a lot of skills on its arsenal. It can hide and counter. Plus it has variety of skills to be used in killing rather than HoP alone (Vital Stab, Harlequin's Greeting, and Pandemonium). This is a versatile class. Though it may have a hard time killing a FD FM on STA gears, but it's not impossible either. I mean you can always hide and then HoP someone. You can counter while HoP is on cooldown. I mean it's hard but being a PRO Harle is worth it.
To sum this up, I'd go with Arcanist because I'm biased. Aside from Arcanist, if you can't afford switching gears, you can try Harlequin. I just prefer FM if someone can afford switching gears. Mentalist, on the other hand, is better than FM but you have to have enough experience to play this class.
PM me here or in-game if you have more questions. My IGN is Shai. Or you can just type %gmlist to see if me or any staff member is online. Good luck.