The strongest FoI itself would probably be Voidsoul purifier - because of rr and extra fire flat and cast speed - but I didn’t test it. The best all-round is paladin followed by deceiver. Surprisingly, deceiver isn’t that good because Voidsoul Doombolt is very bad, and despite loads of chaos flat occultist provides no rr (amulet slot often used for extra chaos rr is taken by FoI wpn dmg conduit).
Here’s a link to my paladin: [1.1.4.1] Devoid Of Soul, In Need Of Buff - Voidsoul FoI/Aegis Paladin (c+) (sr+) Deceiver might be better after the patch, though, as FoI is getting a big wpn dmg facelift.
Why Voidsoul and not Black Flame? Voidsoul got great mods and procs, and Vampirris is a great weapon that Black Flame blocks. HUGE advantage of Black Flame is the offhand and cdr. Shield on chaos is stupid. You’re desperate for that cdr for DG and Abomination uptime.