"The party, _____which I was the guest of honour, was extremely enjoynable."
The answer is “to”，and I think "at" is possible. Am I right?
The only way I can make "to" fit is to add a couple of words.
The party, to which I had been invited as the guest of honour, was extremely enjoyable.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.