I actually like Trip/Shooti the most. I find Trip a fascinating rival for Ash and unlike Paul (who was completely devoid of any human character or relatability), Trip is one of the best things about BW. Sure his attitude is really grating at times, and sure I hate how he thinks he's so cool (a characteristic he and Paul share), but overall he's one of the best things about BW.
Now, for the others and what I feel personally:
Stephan/Kenyan is kinda bland, really. Like he's there to...be there. Nothing too interesting about him, but not a bad character-- although he seems very generic.
Virgil's meh...he's not bad or good to me. See Stephan.
Kotetsu is cool in that he's so different and unique from others around him. He's really enthusiastic about his stuff, granted he's extremely weird at times and comes off as creepy at times. See Stephan and Virgil, I guess.
Bianca is either amazing or a complete ditz. Let me elaborate for just a second. She's either very enthusiastic and funny, or very stupid and moronic. It really depends on the context in which the writers choose to portray her. Better than a lot of the other rivals, but can be really annoying when she wants to be. Good but definitely has her flaws (again so do Stephan, Trip, etc.), but Bianca's are fairly obvious. She has a charm though, and I will say that.
Burgundy is very creepy and annoying, but she's decent.
The one I hate is Georgia, that being with a passion. She's a class A jerk, stealing all of Cilan's food, I could go on about why I dislike her.