Just finished the series, and I must say I was exhausted by the end of it. I really enjoyed the earlier episodes, but as it went on I became increasingly annoyed at the overall teen angst and bad decisions made by the characters. I know they are teenagers, and with a series revolving around them, that is what I should expect, but I thought some of them were quite unbelievably stupid and just serves as poor plot points to drag the story.
I'm with most of your regarding Bryce as the least likable character, but his ignorance towards his actions and complete lack of empathy were just too unrealistic for me. Perhaps it's because I've never met a person like him in real life. If I were to rank the teen characters from most likable and/or sympathetic to the least it would be:
Jeff>Hannah>Alex>Sheri>Tony>Clay>Jessica>Justin>Zack>Ryan>Tyler>Courtney>Marcus>Monty>Bryce.
Based on how the characters were portrayed in the show, Jeff was the nicest one. Hannah was mostly sympathetic, but in terms of likability alone, she'd rank lower for me, maybe between Clay and Jessica. Most of you had pointed out the issues regarding Alex, and Sheri really had that one stupid mistake on her plate. Clay is ranked lower because at times he annoyed me with his overall behaviour throughout the series, especially in scenes that involved his parents and/or Tony. I'm rather neutral towards Zack, Ryan and Tyler. Zack quite clearly was not a bad kid, and Ryan was quite harmless, relatively. Tyler seemed like he is turning into someone dangerous, so maybe he will be the main topic of Season 2 if there is one. He did come clean about Hannah's tapes, so at least there's that.
Not sure if I will be tuning in if there is a second season, though it is tempting to see what actions the Bakers will take next.