Most loved:
Definitely Root – a fantastic character. I thought she was a great adversary, but when Root first got that call in the hospital implying that she'll return I was apprehensive, because I thought they are just going to drag it out and thereby dilute a good villain. However they took her story in a different direction than I thought and somewhere between the middle and end of the third season (with some good bits in the mental hospital paving the way) Root emerged as the most awesome character ever. One of the things I like about Root is her blend of vulnerability and (capacity for) cruelty. So for me Root is a unique character based on that alone, since usually with (in the broadest sense) similar characters entitlement intervenes and so either the vulnerability or the cruelty seem fake and the character bratty. With Root I also like that she doesn't hold back and once she commits she's all in. I like that she can call someone Australopithecine (ahem, I had to google it just now) and make it not seem like a screenwriter was just looking for a fancier word for a typical insult, but in character. I like her intelligence and charm and her wit and that she exhibits both arrogance and humility and all those things make her only more human not out of character. And so on. Basically she can hit any note and be on key. Also, obviously the writing has to be good, but Amy Acker is just amazing.
/ 3. Harold and Reese – with one of them maybe slightly in the lead if either is in a strong episode, but basically I like both and couldn't really choose.
Reese is the strong silent (and occasionally quipping) type, but his sensitivity is more believable than usual in action heroes. All the ladies he's teamed up with for mayhem have been more sadistic than him and Reese has been the reflective one, who feels compassion and is remorseful. BTW one of my favourite Reese scenes is him answering Grace's question about Harold.
Harold I thought likeable from the beginning but it took a couple of seasons before I really started to like him. He is a character that (like Root) could go in any direction and make it work. He has been established as kind and empathic, but the interesting thing is the question how much of an effort he has to make to uphold himself to a high moral code. There is after all still that guy in there too who decided on “irrelevants” being irrelevant and that what matters is scale. So he has a scary practical side to him and I could see his character making different decisions and feeling justified in all of them.
Fusco – he was in beginning a typical slightly sleazy dirty cop but has almost on the sly developed into the guy he is now. Not sure how that happened, since his character hasn't had big turnarounds or revelations and somehow he's been always in character, so his progression has been slow and steady and believable. Have to say I like Fusco's wilful ignorance about the Machine and that he even doesn't try to make sense of it or demand answers, he knows he has the opportunity to do some good and that's what matters to him.
I guess Shaw, sorta – hey, can't give that fifth “Most loved” spot to a villain after all and Shaw did grow on me, even though I think her emotional “limitations” are limiting my capacity to fully care for her and for me she doesn't always seem to be IN character - like I thought it was maybe a too big switch from threatening to kill a guy who's planning to kill himself to getting one clue from Fusco and turning all Roma Downey on Touched by an Angel. So I don't know whether it's the Axis thing that throws me off or I just don't quite get the character, but there are some reservation even though I did come to like her.
Biggest fails (in the sense that for whatever reason these characters didn't work for me):
Carter – I'll be probably downvoted now, but what the hell. Honesty is the best policy, right :P Plus, this post has probably turned out too long for anyone to get to this point anyway :) In the beginning I was neutral and prepared to like the character, but I just couldn't. I gave up for good with the HR arc (might also be a touch of the chicken or the egg there, but I think I wouldn’t have liked that arc with anybody else either). I think the moral compass type of character is in general difficult, because if that character fails to connect, the conceived positive traits transform into entitlement and Mary Sueness. I didn't connect with the character, so that happened for me with Carter. Aaand I refrain from further comments.
The Samaritan avatar boy – he's been in only 2 episodes, so maybe there's a way to salvage that, but if the show stays with how he's been so far, then it's a disappointment. The actor is fine or as you'd expect in the role but I just can't get behind the concept (but I can hope that maybe POI has some surprises in store).
Collier – while I didn't like the character either, he gets a spot here not for the acting or how the character was written, but because his appearance was distracting for me, no slight to the casting agent either. Just a quirk. No offence is meant (though I guess there's no way to express this without being offensive), but just as I can't take Julia Roberts seriously in any role because the teeth distract me too much (thinking back maybe I shouldn't have read Ligea and Berenice as a teenager or thinking even further back possibly even listened Little Red Riding Hood as a bedtime story) I couldn't look at Collier and not think that any minute he's either gonna drop a line about little grey cells (and of course the only Belgian on the show is supposed to be Bear) or don some high heels and move to Vegas. Not really a reflection of the character, but there it is.
No 4 and 5, since the post has turned out too long as it is (and no one else really stood out that bad – though there have of course been also characters I've been more or less indifferent about or have not liked, but I wouldn’t expect anyway that every character hits the bull's-eye).