I think my favorite set will always be the Splatoon trio. I got my WiiU specifically to play Splatoon because that game charmed me so much ever since the announcement, and never once did it disappoint me. Despite not being into shooters at all,or into competitive gaming, I found myself having a lot of fun, and learning the basics easily enough, and I still very much enjoy myself.
After that, I love the Animal Crossing figures for how well they're made. Fire Emblem is a more loved franchise on my list, but the small size of the amiibo doesn't lend itself to detail all that well. I still love my FE amiibo quite a bit, but the Splatoon and AC ones just look better.
Yarn Yoshi are after these. So cute. Too cute. Can't take them out of the box because they're yarn.
I actually did open them, just in a way that preserves the box. Can't clean the dust off as easily since they're not made of plastic, haha.
I gotta give it to the Yarn Yoshi, hands down. I just have a soft spot in my heart for Yoshi in general. He's always been my go-to guy growing up playing vidya games whether it was Mario Kart, Mario Party, or anything, I would always chose Yoshi. I would even go as far to say that Yoshi is my favorite Nintendo character. But back to the amiibo, they're just so freaking tiny and adorable. I could fit all 3 of them into my hand for goodness sake. They're without a doubt my favorite amiibo(at least until Wolf Link/Midna come out heh heh). To close this post off, here is a picture of my Yarn Yoshi on display downstairs for this holiday season.
Of the selection on the poll, I'd probably have to vote for the Pokemon amiibo. Mewtwo's amiibo appeals to me even though Mewtwo is kind of terrible in Super Smash Bros. for Wii U, just because I've been a fan of Pokemon since the first generation and Mewtwo has always had a cool factor. If I ever decided to actually purchase a Mewtwo amiibo, it might make it more likely for me to buy the others (Charizard, Greninja, Jigglypuff, Lucario, and Pikachu) later.
My actual favorite group of amiibo would be the Fire Emblem group. Some had weird manufacturing errors due to the faces back in the day, but mine all look fine. It's my favorite Nintendo franchise, and I like that I have some figures to display that show some of the history of the franchise. Ike is my main, so that's a factor too.