She has personal reason for wanting Rand dead, given what happened to Balthamel at the Eye, and Rand's role there.

She channels saidin, fitting Slayer's opinion of his employer. Of course, we can't be sure Slayer's opinion was correct, but I feel pretty confident that RJ gave us accurate information in that particular case. Just as he did when he had Elayne conclude that Adeleas' murderer had to have been one of the 3 Aes Sedai in their group (Merilille, Sareitha, or Careane).

Aran'gar may have had good reason to hide her identity from Slayer, more so than the other Forsaken. At the time, Slayer may not have known of the 'gars existence, in which case he would likely not have obeyed her had she appeared to him as Aran'gar. The other Forsaken (aside from Osan'gar, who would have been in the same potential position) shouldn't have felt as strong a need to hide their identity, since Moridin lifted the ban on killing Rand in the Tea Party scene earlier in the book.

In her PoV chapter in KoD, Aran'gar thinks about how the Sammael impersonator was playing her game. And while that's not exactly what Slayer's employer did (i.e. the employer didn't impersonate a Forsaken, he just disguised his identity), it does indicate that Aran'gar is sneeky in her dealings with other Shadow agents, as was Slayer's employer.

Also, in the same chapter, Aran'gar made it clear that she had her own plans for killing Rand, fitting with Slayer's employer.

Many of the arguments above can be used to support Osan'gar as a candidate for Slayer's employer, but since Osan'gar died shortly after the scene, I think Aran'gar is the more likely suspect. Demandred is the only other viable suspect, I think, if we assume Slayer was right about the employer channeling saidin. But I think he's the least likely candidate, of the three, mostly because if it was he who ordered the DF Asha'man to kill Rand after the failed first attempt (see below), he didn't feel the need to hide his identity then, so why hide his identity with Slayer?

I've also seen it suggested that it was Aran'gar disguised as Demandred (not Demandred himself) who ordered the DF Ashaman to kill Rand after the failed first attempt, because of Aran'gar's thoughts about how Sammael's impersonator was playing her game. It's possible, but I think it's more likely that it was Demandred himself who did that. We know he and Osan'gar were supposed to be hunting down Rand after that failed attempt, so it would seem Demandred was in a better position to find and order those DF Asha'man than Aran'gar.