My answer depends on what's being asked.

If the question is which of these love triangles do I like best- then I'd say either Wesley/Fred/Gun or Ben/Felicity/Noel because in both triangles I liked both couples. I preferred one (Wesley & Fred and Ben & Felicity), but I still liked the other couples.

But if the question is which love triangle was most divisive, then I'd say either Brooke/Lucas/Peyton or Kelly/Dylan/Brenda.
The OTH love triangle was most divisive among fans. Many many OTH fans classify themselves as Brucas or Leyton fans first before anything else on OTH (before being Brooke or Peyton fans, or Naley, or Lucas fans). And there is definitely this division between the fans and one little negative comment can reignite a war between fan-bases.
I'd say the BH 90210 love triangle was the most divisive among the characters. Sure fans felt really strongly about their preference, but I'd say it divided the characters even more than the fans. Even in the 90210 spin-off many years later, Brenda & Kelly had tension & distrust between them because of what happened when they were in high school with Dylan. Their characters, friendships, and relationships were never the same (unlike on OTH where the characters kind of returned to normal, but the fans never did).

This one is pretty divisive I'd say.
Angel/Buffy/Spike too from what I've seen, and Chuck/Blair/Dan, Nate/Serena/Dan.
Maybe Damon/Elena/Stefan a little, although I'd say - without being biased - that Delena seems to be the one with the bigger fanbase.

This one was definitely the most divisive. It was 50/50. You can see for yourself even here at fanpop, they have pretty much the same number of fans. Also Angel/Buffy/Spyfe.

Delena fanbase is bigger than Stelena. Same for Chair/Dair, even if i ship Dair it's kind of obvious Chair fanbase is bigger (and scary). I don't know about Pacey/Joey/Dawson, i watched DC years later.