Nightmare in Dream Land was a butchered version of the classic Kirby's Adventure, and Amazing Mirror didn't have linear gameplay, making it very hard to 100% complete. Other than those two, all Kirby games are at least semi-decent.
To be honest, I can not think of a Kirby game I played that I did not enjoy. I think that Kirby Mouse Attack, Kirby's Adventure Wii and Kirby Planet Robobot were amazing though!
Favorites would be Air Ride and Return to Dreamland.
Also for favorite soundtrack, it's a tie between Rainbow Curse and Robobot. Both games were just okay but their music was phenominal. (Even though almost all of the good Rainbow Curse music wasn't even used in the game cricricri)
Man, that's a tough decision but I'll have a try at what I think right now.
Favourite: Kirby Air Ride. Arguably one of the best racing games ever made with really good music and gamemodes such as City Trial and, of course, Air Ride.
Least Favourite: ... I can't think of one right now.
I've only played a few Kirby games, but of the small selection I've experienced, my favourite was Mouse Attack (or Squeak Squad in the US). Insanely cute game, I got a lot of replay value out of it with all the collectibles.
For my least favourite, it's got to be Air Ride. For some reason I found this game way too fast to enjoy, which is odd because I love Mario Kart and some other racing games. Maybe it was the overly saturated colours and motion blur, whatever it was made it really hard for me to navigate the courses well. I guess I only played it over one summer so there was unfamiliarity to it as well.
I've played a little bit of Amazing Mirror and so far I've not been able to get into it, I'm not going to bother rating it because I don't know it well enough to really formulate an opinion on it yet.
I forgot about Avalanche and Dream Course. They were way too difficult for me. I'm even getting close to finishing Tilt 'n' Tumble. I can barely get past the second course in Dream Course, if at all. I don't like those games that much due to the difficulty.
I love a lot of the main games but Super Star Ultra will always be my favorite. The new games like Triple Deluxe and Robobot are awesome as well. As for the spin-offs, the only one I liked were Kirby's Air Ride and Kirby's Avalanche. If I had to pick a worse I'd say Mass Attack.