But unfortunately Garp is not available so Vergo should be better option. Sanji would benefit from good CoA tremendously, his CoA has been ass water in New world hence why he keeps eating shit in new world.
I think ivankov was probably a better teacher than Vergo. Ivankov was one of Dragon's top 3, I think he is at the very least YC3 level.
I know that this is if Sanji's training was combat oriented, but I think its important to note that overall, Sanji did receive the best possible training he could.
He literally became the best chef in one piece. He was able to make a cake much better than what streusen made in a fraction of the time. He was able to make frosting that tasted so good that Capone and Big Mom were momentarily unable to move after eating it.
Its important to note that unlike Luffy or Zoro, Sanji's main role on the crew as well as his dreams have nothing to do with powerlevel. Hence it makes sense that his teacher wouldn't be a legendary top tier.
That being said I think Ivankov is also very underrated. I'd rank him at YC2 if not higher. His death wink is basically a shockwave that ivankov can spam simply by closing one of his eyelids. In terms of speed, I think he probably has one of the highest attack speeds of all the high tiers. In addition, I think his DF can be extremely lethal if he used it correctly. he could jack someone's body full of insulin and make them pass out if they're not strong enough.
Its just that Ivankov was put up against insanely strong opponents, Magellan, and Akainu, hence he didn't do so well.