Even though I love oldschool clients (unlike Evan) I'd recommend 8.6, because of it's playerbase.
But as Evan said, it allows people to use Elfbot, however, if the admins stay active and other members report people who bot, getting those botters banned wouldn't be too hard.
Oldschool clients I love, but you have to be really good to keep it alive/get players.
Current client is probably best after 8.6 as it's simply new, new quests, hunts and PVP stuff, mounts and whatnot.