1) Coming in at number one are my Giant Contact bar ends. I find these great for climbing as well as a nice change of hand position on long rides. I have wrapped my entire grip and bar end set up with some extra bar tape to allow some more cushioning and to soak up the vibrations.
2) Closely behind the bar ends are the ESI grips. These are the best grips I have found for comfort during long days on the bike. Basically they are just a big foam grip, nothing flash or ground breaking but extremely comfortable.
3) My must have number 3 are the Maxxis Ikon tyres. I have ridden Ikon's for the last 4 years both front and rear or sometimes in combination with a Maxxis Ardent on the front depending on the condition. I find them so robust and hard wearing through training and long events that I just keep going back to them.
4) At the end of the day your tyres are only good if they have air in them. While I do not get many punctures with the Ikon's when I do I can rely on my Lezyne pump to get me back up and running. Bike mounted pumps have a pretty hard life on a mountain bike and I have been through so many plastic pumps that seem to wear out before you actually need them. Again I find the Lezyne so hard wearing and I have had it for about 4 years and it is still going strong.
5) The fifth and final piece of kit I never leave home without are the Giro Terraduro's. I also ride the Giro codes and find the Terraduro's similar in terms of shiftiness and transfer of power while pedalling but very easy and comfortable to walk in when needed.
So there you have it those are my must have accessories.
As a side note. While I am a T7 sponsored rider I purchased all of the above items and choose them from their extensive range to meet my needs.