I'll try to give it a shot:
I think "slice of life" stories on either side of the Pacific (ie. Japan or North America) tend to towards two unfortunate extremes:
1. the protagonist is SOOO incredibly bland that the they're nothing more than a reader surrogate: the main character is essentially a mask that the reader can put on and wear to immerse themselves into the world. typically they get overshadowed by the supporting cast who are intentionally strange (and therefore more entertaining and memorable). when the main character is purposefully nothing more than a mask for the reader to wear... it's not hard to see why some people would hate the main character of so many anime and manga.
2. writers overplay the characters' vices to make them more "real!" sometimes this gets done so much that you can't really LIKE the main character. now, granted, for some people that's perfectly acceptable. (basically, some writers "edgelord" their characters so much that people just don't care anymore)
now that I think of it-- it's exceedingly rare for my favorite character in a shonen title to be the main character! the main character is usually bland to make them into a more suitable reader surrogate. you can have genuinely fun and memorable characters like Goku or Luffy... but even here it's possible for them to get overshadowed by their supporting cast.
the only anime/manga I can personally think of where the main character was actually my favorite was "Ranma 1/2". (huge character flaws and all. for all of his vices he had some legit virtues to counteract them. he was also pretty funny)
I realize that I didn't give any specific examples... I figured that would probably be better left to Nate Grey. I could take or leave most shonen protagonists. they all start blurring together after awhile.
Friends - Rachel and Ross. The whole will-they-won't-they bull ruined the show, particularly later on IMO. Particularly when Ross was the central figure of the episodes as I couldn't stand him, at least with Rachel I had a crush on her so she was somewhat bareable to me at the time LOL.
C.S.I - Catherine Willows - Never liked her, she seemed a bit too...Snarky? I guess? Anyway, I never took to the character personally speaking.
Star Trek: The Next Generation - Wesley Crusher - I just thought Wheaton was a rubbish actor back then and it didn't help that, particularly in season one, his character basically made all the senior officers look incompetent at times.
The Bill - Samantha Nixon - another character I just couldn't take to. And to make matters worse for me she stayed in the show for years and was a big star of the show so she got lots of air-time and took away from characters that I did like. Doesn't help that she came into The Bill when it had started to go downhill and never really recovered (aka the ''sexing up'' of The Bill)
Arrow - Laurel Lance - I just thought she was horribly mis-cast and, I'm sorry as I know she has her fans but I think Cassidy did a horrible job as an actress in the show for the most part and I was happy when she got killed off.
Scrubs - JD - Like someone said about Ally McBeal, it's a strange thing to not like the star of a show but liking the supporting cast but that was the case with Scrubs for me.
Anyway, I think that's enough for now
Totoro Man nailed all the problems I have with slice of life protagonists. I'm not the type who enjoys reader surrogates as I don't want to inject myself into a character.
As for shonen, I really dislike Goku, Naruto, Kazuma (s-CRY-ed) and Natsu (fairy tail). They all fall into the idiot hero archetype and it's hard for me to enjoy a character who's main attribute is being a selfish moron. I think Yusuke Urameshi and Kenshin are the only shonen protagonists I like.
Xander from Buffy The Vampire Slayer. Horrid little creep 90% of the time.
The 10th Doctor on Doctor Who. Grinning idiot. He just needs a palette switch and he's a perfectly fine Joker. He's even got a body count to match.
The proto-Riddler on Gotham.
Gan on Blakes 7 (yes, that is the correct spelling).
Lance Hunter on Agents Of SHIELD.
[QUOTE=Carabas;2460676]Xander from Buffy The Vampire Slayer. Horrid little creep 90% of the time.
I don't know if you read the comics but he's also in a relationship with Dawn as well now.
https://youtu.be/ER7bIGwJeho
I was hoping we got rid of her. Sigh. Oh well, at least she won't be the movie version since apparently they're casting for Black Canary in the films (no idea which one she'll be in but I'm guessing JL)
I personally thought he was ok in Buffy. Wasn't my favourite character but ok. Then again this thread IS about our least favourites so I'll end it there LOL.
I just wasn't sure whether or not you read the Buffy comics, so I thought I'd mention it in case you also had a problem with him dating Dawn.
Buffy really ought to have ended at one of two places:
1. when she killed Angelus to save the world.
2. when they graduated from high school.
the biggest problem I had with Buffy is that the show soldiered on long after it's "sell-by-date" and left a bad taste in my mouth. the writing quality dropped off considerably after Whedon migrated to Angel and basically farmed it out to others. the balance of action, character, drama, and humor never really recovered. they also started "writing for the moment". this typically ends up at cross-purposes with good characterization and story-telling in the long run.
by the time I stopped watching Buffy I didn't really like ANY of them.
Raylan Givens in Justified.
Don't get me wrong, the show was amazing, and had excellent acting all round, but by the final season Raylan's pulled all kinds of ****, and never once gotten any meaningful blow back. Worse, he's leveraging the girlfriend he cheated on (and should know is innocent) to provoke Boyd into committing a crime, while ignoring the greater threat in Markham
I think it was more of the writing that let the 10th Doctor down, Tennant was good in the role. Rewatching some of the RTD era a while ago reminded me of how repetitive it got (every series has the same structure). It introduced me to Who but I greatly prefer Moffat's stuff now.