Sanji's is the one I'm most stoked for. Both as a fan and because he has a variety of potential opponents.
Captain of the First Ship Jesus Burgess with a strong fruit could make for a formidable opponent. He's one of Blackbeard's first crew-mates so him ending up really strong wouldn't be surprising. A few losses can be redeemed and him getting focus in Dressrosa atleast could suggest he'll be one of the most important Titanic Captains.
Avalo Pizarro has some qualities that could make him a good Sanji opponent. They're both former North Blue royalty so there could be Germa similarities though perhaps that's getting overdone (hopefully the angle it's taken is interesting if it happens). Perhaps stuff like gluttony against compassion and such.
Laffitte is the Chief of Staff so he holds the same position that Sabo does in the Revolutionary Army so maybe. I'm not really crazy on this match-up. I'm sure Oda would do something cool with it but I prefer the other choices. Laffitte against Nami seems a pretty nice option too.
Tenth Titanic Captain is obviously another option since it allows Oda to fit them wherever without much hassle.
I've heard people mention other choices but I'm not crazy about those and these seem the most likely to me. Whoever Sanji faces, I look forward to it.
Also, Luffy vs Blackbeard
Zoro vs Shiryu
Usopp vs Van Auger
Chopper vs Doc Q and Stronger
Nami vs Laffitte/Devon
Robin/Yamato vs Catarina Devon
Franky/Jimbei vs Sanjuan Wolf/Jesus
Vasco Shot vs Brook or whoever cause that guy hasn't done much
10th Titanic Captain against Yamato maybe