We have not yet been to Kona (we are trying breakfast in September); we LOVED Yak & Yeti when we were there in 2008; you could not pay us to eat at WCC again -- awful service and awful food.
IMHO, none of those are worth going the extra distance for. I like all of them and we had Yak & Yeti just two weeks ago and it was great, but if I was not doing AK that day, I would not do a park hopper to get there. So, my answer would be whichever of the 4 you are closer to on the particular day.
I'd say Kona Cafe or Chefs de France, based on personal experience. Both are solid, tasty, and fun. Chefs is moderately more romantic as a restaurant, in my opinion, but the Poly as a resort is quite romantic
We've never eaten at Yak & Yeti TS, so I cannot vouch for or against it.
WCC is the only restaurant in WDW that I won't return to, and one of the least pleasant meals of my life. Many love it; I certainly do not!
But of the other 3, Yak and Yeti has the best food. I go there for lunch every time I'm in AK. It's addictive!
Kona comes in second for me. The food was good, but not as good as Yak and Yeti for me.
And Les Chefs...well let's not even go there. Literally.
Kona by far.Best bread in Disney.Good food&awesome desserts esp the creme brulle
We love Y&Y but it's not the same class as the other 2.
Chefs is nice but a distant second in France to Bistro and not significantly less expensive.
Kona has great food but lacks atmosphere. Chefs de France has good food and a romantic atmosphere. We will not be going back to whispering canyon--kiddie atmosphere and the food wasn't very good. Going to both Kona and Chefs is a great choice.