My List is likely to be unpopular. I omitted people we know are dead (for the time being, as it's likely Monica will soon be added to that particular list once we figure out what happened to her), but kept those we know who are alive, but not appearing atm.
Like
Peter Petrelli
Nathan Petrelli
Noah Bennet
Claude (More Eccleston please!)
Elle Bishop
Ando Mahashi
Matt Parkman
The Haitian
Don't Like
Hiro Nakamura
Angela Petrelli
Monica Dawson
Daphne
Micah Sanders
Adam Monroe
Tracy Strauss
Can't Stand
Maya Herrera
Sylar
Mohinder Suresh
Claire Bennet
Totally Screwed for No Good Reason Currently
Caitlin
One of the biggest reasons I don't like Sylar is two-fold (and applies to Peter as well, yet Peter to me is more likeable because his intentions have been consistent), and it's from his rapid collection of powers. Assuming he regained all of his abilities from Season 1 (which seems to be unlikely, as he commented when he attacked Claire that he wasn't "completely" better), he would have a massive and unstoppable array of abilities. But, if his powers hit a soft reset after the Shanti virus (leaving him with his Intuitive Aptitude and Telekinesis) then it's only a matter of time before he once again gets enough powers to become an unstoppable juggernaut. Save for Peter, that is. I think one of the only reason Peter is still around is because of Sylar. There needs to be someone equally strong enough to provide a foil for him. I'm not big on Peter gaining just as many abilities as Sylar (because again, you have one unstoppable juggernaut, and as is the case with Superman in the movies, it makes it hard to create scenarios where he can't just waltz through them), but I find his character's struggles to be more genuine than Sylar thus making him less cliche'd.
And Sylar turning good? Don't see it sticking, and unfortunately I don't think he'll ever see the consequences of the damage he's wrought. Which is kind of sad because his Storyline seems to just repeat itself over and over. He's a killer obsessed with getting powers. He's "good" for not even a whole episode, and he's whackin Jesse for his powers. To me, if Sylar winds up getting away with all of this, the series will be a let down for me, despite whatever happens with the rest of the characters.
While it's true, not every villain (depending on your perception) in the real world gets his comeuppance, I think it devalues the entire struggle of the people in this series if the biggest antagonist in this series doesn't get his due. Sure, they could save the world dozens of times. But what happens while this maniacal power-mongering villain is still on the loose? I dunno.
/rant