I've been to so many weddings that haven't been great - some of that is me; all the dressing up / getting hair done etc takes hours, and I love the church part (I think all the weddings I've been to have been in churches as it's only quite recently you could get married anywhere other than church or registry office in Ireland, and most registry offices were NOT nice). I find I've run out of steam once we are half way through the meal. But sometimes i get a second wind if the music is great.
So best parts: lovely music in the church, and how fabulous everyone looks; when there are decent canapes / snacks provided between church and meal; good sandwiches / cake for the afters (is that a thing in the UK? it's where people are invited to the evening reception part, after meal etc and about 11 p.m. it's expected that afters food is brought out, often including sandwiches with some of the meat served at the main meal, slices of the wedding cake etc and loads of TEA!)
Worst ... oh God so many. There was a good friend of mine who was marrying a girl who was very very shy and desperately insecure about their relationship. I never understood why they got married (but they're still together so...!). Anyway, at the altar, when it came to the vows, she just stopped. Said nothing for an age. You could hear a pin drop. Eventually, she turned and walked away, leaving the groom at the altar! Everyone sat stock still. Priest, bridesmaids went after her, and a while later she came back, said vows, all done. No-one said a single word about it!
That wedding was dull as ditchwater. Everyone ate their meal, quietly. Almost no-one went on the dance floor and all wrapped up I'd say by midnight.
Plenty of drunken messes, especially when it came to the speeches. I remember one best man, drunkenly making his way through a list of the groom's (alleged) past sexual conquests. It just got more and more vile, and seemed never-ending. The look on the parents' faces at the top table ...