In order to make a significant impression on me, a crew member (as opposed to the Disney-esque 'cast member' -- notice off-limit areas at USF say "production crew only"') must perform beyond what they're expected to.
I think most Universal and Disney employees do their jobs very well, are very nice and personable and informative, but I also consider that part of their job at a theme park. An outstanding crew member goes above and beyond what's expected of them, where it seems like what they're doing is more then just their job.
My vote goes to Courtney at the dog kennel. I recently used the kennel and at first felt bad about leaving my dog and unsure about how the whole thing worked, but as soon as I met Courtney her entire attitude and treatment made me feel completely comfortable. She wasn't just nice and thorough; it seemed like she truly cared about the animals she was entrusted with and understood their owners apprehension. It seemed like it was more then just a job, that she went the full length because she really wanted to.
My honorable mention would be Vinod, the turnstile attendant at Islands of Adventure. Granted, he started being nice to me after I felt he was rude to me and I complained to guest services. Then again, he started being nice to me because I started being nice to him after I felt bad for complaining about him to guest services. Never the less, we now greet each other when we're both there, and he's genuinely personable to me as an individual.
-Kevin