if this is limited to drugs i have tried, then i would say an unlimited amount of alprazolam and heroin. i actually cannot stand heroin or any other opiate for that matter but i think with an unlimited amount, my body might be able to grow accustomed to it and then i would start to get the euphoria without the horrible side effects i get now.
if i could choose drugs i haven't tried, then i would go with an unlimited amount of quaalude and seconal.