At the ceremony
Reception Venue, Photographer, Videographer, Flowers, Make-up, Jewellery, Wedding Dress, Brides maids dresses, Limo, Wedding Cake, table decorations, suit hire, celebrant, ceremony venue, DJ/Band...
No question - in the whole of your life, it's the most organisation you'll have to do for one day.
Picking providers can be difficult. For some providers you can see examples of their past work, do interviews or actually see them in action, however that can be a very intensive exercise, especially if you are diligent with every provider you use.
One way to cut down on the number of people you have to see, is to hit up the people whose services you have already engaged for people they recommend.
In the case of someone like a photographer, they will have seen many weddings and will have seen many wedding service providers in action. If they do recommend someone, chances are they will be good. After all, they aren't going to want to be remembered for the bad job someone else has done for you!
So if you have someone you trust who you have already hired - use their experience. It might cut the time spent looking for other services!