Banana, from how I play Heavy I get hit alot, so I need to heal alot. 100 hp is very satisfying and enough for me to wait less than 5 seconds for a 100 restoring healthkit. If you have a pocket med tho, the Sandvich. You need a bigger healthkit for your med.
in-depth :
Without Medic girlfriend : Banana, Faster recharge allowing you to give more teammates healing when there's no med and you can still eat it yourself and it recharges very fast.
With a Medic on the team : Sandvich, Better healthkit if your med needs healing, and better healing for you, use it for emergencies, like if your med is dead. Heal some teammates with a medium healthkit amount of health until the med respawns to heal the team.