Definitely Gyarados. It have above average typing against many top threats of UL (non-dragon to Giratina, resist all Swampert&Registeel moves, resist fighting power house like Lucario, resist fire starters and neutral to grass starters and popular ice moves).
On the other side of the scale, it’s only double weakness to Electric is seriously underrepresented in UL, practically doesn’t affect Gyarados. Melmetan is the only relevant electric user in UL and still, he will probably just RockSlide you to dead instead. The great movepools of both fast and charged moves are unrivalled to most too. The only issue here is that it is very glassy and can’t take much beating even from favourable matchups. I’m using PVP IV ranked 221 (CP2493) Gyarados for reference.
Dragonite is pretty much on a suicide mission with its Flying + Dragon typing. Giratina is everywhere and ready to eat Dragonite for breakfast, lunch, dinner + some left for suppers. Those icy counters to Giratina also gonna having their field days. Pink fairies will thank you for letting them charge up the MeteorMash/PlayRough/ShadowBall for free. Registeel laugh at you with their Flash Cannon. Your only consolation is to take some beatings from mudbois and grass starters.
P/S: just run a simulation of Dragonite vs Swampert, and to my surprise, Swampert manage to win in one shield advantage, another big blow to Dragonite (as if we hadn’t have enough of it already, LOL).