I don't really like Mars bars that much so tend to go for snickers, or something flapjack based like a cereal bar, I have often taken pepperami too. Personally I swear by orange lucozade, have a few swigs before starting prusikking for an energy boost.
As mentioned above, Kendal mint cake is awsome and they do a lovely liquorice flavour one too. When I started caving I used to take a bar of mint cake underground and share it out with everyone but I don't do that anymore! It started to hurt my teeth and nobody returned the favour so I realised being selfish was the way forward with food underground!
If I have time to plan ahead and not just stopping at the garage on the way, I take dried fruit mixed with nuts. Apricots, mango and pineapple squashes down quite small. I use good quality zip lock bags, cram whatever I fancy in them and then squash all the air out