I kind of agree with Flik-E in that the rejects don’t really have enough screen time to correctly display their personalities, but I seriously love them all just because they’re so different and definitely underappreciated.
Well, if I had to choose I’d have to say HAN-S. I dunno why, he’s probably the craziest out of them all, and I just crack up so hard whenever he’s doing something on screen. 
little chef
I like BRL-A, L-T, and VN-GO (because he paints like me!)
Since he hasn’t been said yet, I love VAQ-M. It’s such an inventive idea by PIXAR to have a vacuum cleaner with allergic reactions, and out of all the reject bots I think he looks the most realistically possible.
I voted HAN-S
I like when he [spoil] totally massacres the steward bots! [/spoil]
I like the broken PR-T bot, I have no idea why, and I’m an art person, I like the paint bot too. 
I love PR-T. I’m not sure why. I just love her.
I love M-O, too!
I love VN-GO, VAQ-M and PR-T. But they’re all adorable anyway.
I think VN-GO is by far the best
Han-S. He’s the best, you know…
PR-T is my favorite. I love her voice: “I know honey, I know!”
She’s the funniest, I just like Han-S’ attitude. Excitement and violence. Perfect.
VAQ-M
In the WALL-E soundtrack, its weird noise thing starts up in the middle of “Repair Ward” and I find it rather out of place and at the same time very awesome.
I voted for BRL-A because of a really good fan-fic I read.