If you want slow release energy the higher fibre items will be better, less processed and will take your body longer to break it down. I would expect brown rice to be the highest in fibre out of these?
Porridge is also good, with unprocessed oats, not Readybrek!
Instant energy releases can help, things you can get into your system fast if you have a sustained climb approaching and feel a bit jaded. Jelly babies, flat Coke, Chocolate etc. Give it time to work though 10-30mins.
I find flapjack is the best of both worlds. Has a sugary but not to processed punch and also the slow release energy of oats. It also fills you up, doesn’t melt, tastes good and is easy to carry in a pack.
Combine this with a healthy breakfast, porridge etc, lots and lots of water and you should be good to go all day.
May also be worth looking at electrolytes for your water, I used these in Spain when it was in the mid thirties each day and I was losing a lot of salt.