1 Littlefoot
2 Littlefoot
3 Littlefoot
4 Littlefoot
5 Littlefoot
6 Littlefoot
7 Littlefoot
8 Littlefoot
9 Littlefoot
10 Littlefoot
Welp.
I can't say I hate any of the characters, but here's my least liked:
10. Red Claw - weak villain. He hardly poses a threat to the kids, let alone any other Great Valley residents. "Biggest and meanest sharptooth of them all"? Yeah I don't think so.
9.
Veeery specific here, but that longneck in the Old One's herd who immediately went, "NOT ME!" when Grandma asked if any of her cousins were willing to take her to the land of mists. Like, geez, the guy didn't even hesitate to shut down the lady begging for help to save her dying husband.
8. Wild Arms - I do think he has comedic value when he's with Grandpa and Topps, but as someone else stated, he took Shorty's role. I can't forgive that easily, sorry.

7. Dinah and Dana - They're just the younger-and-more-annoying-than-the-main-kids trope, and yeah, they're certainly more annoying.
6. Petrie's siblings - Constantly on Petrie's back and very judgmental overall, and their voices are kinda annoying.

5. Ozzy and Strut - An entertaining pair at first, but they have an unhealthy obsession with the kids that directly stopped them getting one egg. And Strut's decision on what to do with Littlefoot at the end... yikes.

4. Tippy's Mom - She comes across as overbearing when it comes to Spike and seems to have no qualms with separating him from his family. And as RainbowFaceProtege said, she comes across as pretty fake-nice around Ducky's mom which really rubs me the wrong way.
3. Mr Clubtail - He's just there to be grouchy most of the time.
2. Hyp, Nod and Mutt - Every time they appear on screen it's just 'I'm bigger so I'm better than you' and it's the same formula in every scene they're in. They show up, threaten the gang, then get scared off by something.
1. The yellowbellies (yeah, original) - Actually... Foobie's okay, because I like his bond with Spike.

I'm not sure how any of the others survived in the outside world.