Hipflask will most likely contain some Islay goodness. Ardbeg/Lagavulin most likely.
Ideally I'd have 3 hipflasks though:
1. Malt as mentioned above for sampling at the summits and general sharing good times.
2. Absinthe if things get really bad. To trigger hallucinations that hills aren't as steep/legs aren't as tired/bike isn't as broken as I thought.
3. Rohypnol solution to induce collapse/illness and hopefully a heli-lift off the hill. Recovery from a revitalising hospital drip then get to the nearest bar ASAP.