Quick reviews in order of release, action/adventure games only.
Kirby's Dream Land (Game Boy) - Excellent. Pretty straight forward game with tons of charm. A little slow and easy, but upon completing the game (or visiting GameFAQs) you're given a code for a hard mode, which provides a decent challenge.
Kirby's Adventure (NES) - Excellent. Adds Kirby's ability to steal enemy powers, faster gameplay and a sort of overworld, letting you go back to levels you've already cleared. Decent challenge and a fair bit of secrets.
Kirby's Dream Land 2 (Game Boy) - Excellent and my personal fave Kirby title. A mix between DL1 and Adventure's gameplay. Adds animal friends Kirby can ride, which can also steal powers creating even more options. Decent challenge (and finding the items to unlock the true last boss/ending can be pretty damn hard if I remember correctly). The animals make it a little easy, since they have their own powerbar, so try not to abuse them.
Kirby Super Star (SNES) - Excellent. A collection of mini Kirby games that range in styles. Some are classic platform/adventure games, some are silly things like a race with King DeDeDe. All are fun. The main change to this is bad guys can now take more than one hit in most cases so it spices up the gameplay a bit and you can not only steal enemy powers, but you can also clone them to fight along side you. The challenge in this one is mixed, depending on the game, but there's a ton of content to keep you busy.
Kirby's Dream Land 3 (SNES) - Poor, the black sheep of the series. The faster speed of the later games is completely sucked out of this one and replaced with a crayon sketch art style, specific to this game only, which looks pretty good but doesn't seem very fitting. 3 new animals are added to the cast along with the original 3 from DL2. There's a decent amount of levels and a lot of hidden items to find, but the bland level design and slow gameplay really hurts the title.
Kirby 64: The Crystal Shards (N64) - Poor, the only title on this list I couldn't play to completion. Everything bad about DL3 returns. The 3D graphics are kind of neat, but not really needed. I recall it being a bit harder than DL3 though, so that's a plus.
Kirby: Nightmare in Dream Land (GBA) - Good. A port/remake of the NES title with some added content. The new content isn't really all that fantastic, but the updated graphics and unlockable character (that really changes the feel of the game) are a plus. Much, much easier than the NES title though.
I can't comment on Kirby & The Amazing Mirror (GBA) since I haven't picked it up yet. The change to a 4 player team whatever doesn't look all that interesting, but I'll probably pick it up since I own everything else I've listed.