I am a 116 Adventurer and I solo xp 95% of the time and would like to add my experience since I spend a lot of time optimizing my hunting.
Approximately 50% of my solo mob kills are not after shadow chase due to its cooldown or if I am very close to the next mob. (Note, I am pretty much maxed out on cooldown reduction.) Those non-shadow chase stabs are not from the rear most of the time obviously because the mobs don't always spawn with their backs to me. (For that reason, I don't even use backstab in my macro when soloing. Reverse Mana resets the other stab cooldowns improving this. I don't think ghost hunters or spoilers get this stab reset ability.) With my current damage, the level 118 mobs at wastelands usually still die in one hit, front or back. However, it's close from the front. If I were a ghost hunter, the lesser front damage would ensure that I not 1-shot the mobs from the front.
So, if conditions are right, an adventurer will kill faster. This may apply to spoilers too but I don't think they have the stab cooldown reset from reverse mana that adventurers have. I'm not sure how much of a difference that makes.
Note, if I had Hyperlite's level of gear I would probably one shot these mobs from the front anyway and there is no higher level soloing zone. DV doesn't count, even Hyperlite can't hunt there efficiently. Therefore, ghost hunters are faster if you geared enough to 1-shot everything at the highest level. However, most people aren't so I think it's valid.
TLDR - If you one shot everything from the front or back, ghost hunters are definitely the fastest killers. If you don't one shot everything from the front as a ghost hunter, adventurers or spoilers may be faster because not all stabs are from behind when soloing. In a party, ghost hunters should be the best regardless since most stabs should be from behind.