Salamence and Garchomp are my favourites. As a kid, I thought Garchomp had the coolest design among Dragons I still think the same and has a unique type combination. It was a dream come true for me when I finally caught Gible because the Garchomp from the anime stoked me, and I remember praying that I get a Gible ( I didn't know the under-the-bridge part of the cave and wandered to no avail in the other section of Wayward).
Salamence has an interesting design, and its story of evolution is inspiring - how the flightless Bagon dreams and eventually becomes the sky soaring dragon! Also, it was the first fully-evolved dragon type I had, so there's some bias here.
Metagross is cool and all, but I never used it in a team, unlike the other two I mentioned, whom I've used continuously in different teams because I can't get bored of them! Would have used Hydreigon had it not been such a late evolution.