Rayquaza! Wut. Very overrated, but seriously it deserves it. Especially it's shiny colors. It was the only member outta the trio that had a HM in it's movepool (not really sure), and it had Outrage. This move... it could knock you out in the first few turns, and possibly the whole team in around 7-8 turns, maybe 6 OKHOs if you're underlevelled. Also, Sky Pillar is amazing ftw! As a kid I spent what... like an hour or two trying to get to the top? I hate but also love how you have to ride the Bike in a certain way, otherwise you fall to the bottom floor. You feel amazing when you reach the top, then you have to fight Rayquaza. I applaud it though, for actually being difficult to reach! Only a few legendaries are like this, luckily they didn't make it even harder haha. I don't know why but I always thought you could only catch it in a Master Ball, so I didn't even try an Ultra Ball :')
I've always had an affinity for water type Pokemon, since I was a child I loved water animals irl, so Kyogre has been my favorite, even though Rayquaza is certainly awesome in its own right. Groudon never struck me very well though, it always looked overly bulky and just didn't seem like a great design.
I love Rayquaza's design a lot. I might be a bit bias because Emerald was my favorite game and I never played Ruby (and never got to experience Groudon much ingame) and I love dragons, but whatever. He is hella cool.