Regice has always been my favorite, even now with the reveal of these new ones. cool cry, was very useful in battle for me in the rse days, and just a neat design overall. love them all though! these guys are awesome (b' ')b
Mine would definitely have to be Regigigas. I attribute it to Generation 4 games being my first, but I spent a lot of time at Snowpoint Temple as a child trying the ice puzzle, only to be unable to receive Regigigas. Maybe some would find it sad, but it honestly just drove me into liking it more and being fascinated by it.
Regigigas, because he's supremely awkward to get a hold of and terrible in battle - and he thus embodies everything the Regis are all about. I love how obscure they are, I love the fact that RSE offers virtually no signposting at all, and I even appreciate the fact that the trio are competitively lacklustre compared to their fellows. It's all about delayed reward, and the difficulty in obtaining them makes even a mediocre prize very sweet. It's fantastic worldbuilding, and Regigigas is the cherry on top.
I like Registeel's sleek, robotic look. I think it was the only one of the OG trio I was able to catch in Gen 3 when I was younger.
While I don't particularly like Regigigas's design, I do think it's relationship to the Regis and its method of unlocking is super interesting. It's definitely one of the better legendary quests in the series, if not the best.
Regice; crystals are inherently cool (pun intended), and the cry is pretty pleasing.
I also like Regidrago even if it isn't released yet; the dark red/black coloration is a nice and edgy palette, and I do like the aesthetic of it being a crystal robot with the gaping jaws of a dragon as its limbs.