Dialga.
Palkia's sweeping capabilities, though monstrous, are pretty much outclassed by Kingdra as a whole as Kingdra gets to exploit Swift Swim. What it has over Kingdra is Focus Punch which might snare a Blissey on the switch, and a stronger Aqua Tail versus Kingdra's Waterfall. But ultimately I feel that in a speed-based metagame such as Ubers, Kingdra's higher speed gives it the edge here.
Dialga has a great movepool combined with better typing than Palkia (in my opinion). The MixDialga set is by far the most common and deadly as it posesses the ability to open up holes in the opponents team by abusing Draco Meteor and Brick Break.
Dialga also gets an immunity to Toxic, which gives it an edge over Palkia as it garners more free switchins wherein Palkia would be crippled. Its typing, Steel/Dragon, means that it can switch into many pokemon in Ubers and with its immense list of resistances, shrug them off.
Palkia may have the speed advantage, 100 versus 90, but keep in mind that Dialga is primarily played as a bulky tank, not an all out sweeper like Palkia is.
