Fencer is a standalone class (Because we have 1 0CD Skill to spam, and with Epee Garde up ur dmg is incredibly high - that is with good gear)
Also dont forget u will need a lot of SP Potions…
U have to be Fencer C3 for Epee Garde, so R6,7,8 are out.
Means u are free to choose between R2-5
Here are some preferences.
Sw3 is pretty good, cuz Pain Barrier lvl 15 is very strong, also Gung-Ho buffs Fencer dmg very much - cuz we lack any form of good rapiers, so PAtk is very important for us. (This is not the case with Lolopanther or Solmiki Rapier, tho)
Peltesta is a good Choice, Guardians additional PDef and Evasion is pretty good. But u wont ever use a shield, so u only take it for that and maybe Swash Buckling.
Barbarian is a solid choice, cuz u get Cleaves additional Crit Rate, and a slash Dmg debuff furthuer increasing Flannconades dmg, if u need some slash dmg, while ur Epee Garde is not up.
Higher ranks barbarian are also pretty good,cuz Warcry increases ur PAtk by a lot.
Hoplite C1 is very nice, cuz u can use Stabbing and Pierce (Stabbing is incredible strong, cuz u have a lot of hits and u can double the dmg with Preperation)
Corsair C1 is also a nice choice, if u like some more CC with Hook and Dust Devil - also u have the Pillaging
So given those choices there are some popular builds:
Sw3-Pelt-Corsair-Fencer3 (This is my build, but if i ever can i will change Pelt for Barb1)
Sw2-Barb3-Fencer3 (Some more dmg, but i realy love this Pain Barrier lvl 15…)
Sw1-Pelt1-Barb3-Fencer3 (same as before just without Restrain and with Guardian)
Sw3-Hop-Corsair-Fencer3 (High Pain Barrier, Stabbing, some CC)
Sw3-Hop-Barb-Fencer3 (High Pain Barrier, Stabbing, Cleave)
So this are the Fencer builds i would choose from. For my playstyle the best is Sw3-XXX-Corsair-Fencer3 (This is because i realy love the Corsair Outfit and the Hook for some Pvp, also i can never go back from Pain Barrier lvl 15)
Hope that gave u some little insight on Fencers, u cant realy pick anything wrong, cuz ur main Skills are from the Fencer class itself, everything else is just additional bonus - choosen because of preferences.
The future rework might change all those things, but without the real patch notes its just specualtion.