I guess I'll pick return to dreamland by default. There hasn't been a really good kirby game in a long time. The three listed are all pretty meh.

epic yarn because it is charming
but haven't played last one

Epic Yarn, because it has that distinctive charm that makes it so cool!

Can't say much about the others but I found Epic Yarn to be quite boring.

Epic Yarn was different, enjoyed co-oping with my non-gaming sis, not being able to die was a huge plus for her. The only questionable thing was the "story".

Return to Dreamland didn't interest me when released, and no Rainbow Curse here yet.

Epic Yarn is really awesome.

Epic Yarn was a actually a bad ass game for it's originality, the full integration of the "yarn" concept, the great soundtrack, good gameplay, etc. Many people complained that it was "easy", forgetting that not every game needs to be Ninja Gaiden hard. I really enjoyed collecting the beads, and the post-level challenges are often tough extras to go for. Wario Land and Epic Yarn both convinced me that Good-Feel is one of my fav. new developers, and I can't wait for Yarn Yoshi.

On the other hand, Return to Dreamland was a great game and had a lot of fine points. I honestly prefer sprite-based side-scrolling platformers, because they just almost always PLAY better (better, more exact physics, less "floatiness", etc.), and I do think RtD would have been even better if they had kept that awesome hand-drawn look they had experimented with at one point. But, it is a great game still, and was very fun, it was basically the GC Kirby that I had waited so long to finally come out.

I'm not sure which one I'd say was "better", I enjoyed them both, and consider them both very good games. Two of the best released last gen.

As for Rainbow Curse, it has a fantastic art style, and I have enjoyed it so far. I need to play it more to fairly assess it, but it's a good game. I just prefer the other two, more my style.

I haven't played the other two, but I will join the choir in saying that Epic Yarn is indeed a badass game. Boring? Really? I call it eye candy. So innovative on so many fronts. Just performing Kirby's move set is a joy. And some of the platformer elements should be emulated by other developers. I too am looking forward to Yoshi's Woolly World, and I've never even played a Yoshi game before. Also hadn't played a Wario game that I liked until Warioland: Shake It...so I guess that means I really like Good-Feel.

Kirby's Return to Dreamland.

Although I did enjoy Epic Yarn, but it was a little boring and VERY easy. Though I assume it was intended that way. My friend and I finished the game with no troubles.