Which Dragonite Prestiger should I go after?

Piloswine is probably your best bet in terms of effectiveness. Powder Snow / Avalanche is a pretty OP moveset.

According to this spreadhseet, dewgong is the most CP efficient due to its tankiness, and it also resists steel. I think dewgong would be optimal, but jynx or piloswine would probably be easiest to get.

CP efficiency spreadsheet: https://www.reddit.com/r/TheSilphRoad/comments/5v88qm/cp_efficiency_spreadsheet_updated_with_gen_2/

If there is a nest of one, go for it. Jynx especially. You can catch a 1300 cp Jynx with double ice moves and you are good to go. I use cloyster because shellder were very common very my girlfriend lives. Piloswine is good, but I have been a little unlucky with moves for that one so far.

Ultimately the moveset and dodging matter more than specific species, in my experience.

Anything with double ice moves is good. As noted above, easiest method is to find a Jynx nest and evolve any of your smoochems.

I've got a pair of ice mons at 1400, 1700, and 2300 that I use all the time.

Dragonites are actually the easiest mon to prestige against.
Pick almost any mon and have fun.
It depends on what mood I'm in. Vape, Wiggly, Clef, Jolteon, etc pick anything, they all work.

In my area, Lapras, Cloysters, Jinx, Piloswine, and Dewgongs are very rare. But I find Azumarill to be the most common in my area and he's a pretty good prestiger against Dragonite if it has Ice Beam. He's tanky too.

Dewgong gets my vote if you can get enough Seel. Unfortunately it can roll with steel and water moves now, so a caught ice/ice Jynx will work, even if it has very low HP