Rahool, on the principal that Xur doesn't hide what you're getting (save those ever exotic engrams), but may disappoint you on a weekly basis as he may not have the exotic you want (Hard Light) or has an exotic you absolutely wish he wouldn't sell (talk to a Warlock, they know what I'm on about).
Rahool, on the other hand, is a crap shoot at best. At least he won't downgrade you anymore...
If this is :
Who do you hate the most?
Then that title easily goes to Rahool. Xur is occasionally helpful. Rahool has thrown me 2 scraps in 4 months and I would like to beat him to death with his own fist.