What is the best rum one can buy on Mallorca?

That's a tough one. I've been to mallorca twice and didn't see much in the way of rum beyond the standard party brands. On mainland Spain I did find some cachique rums which have good reps, you might find some in mallorca. I also found some good French agricol rums which might be worth getting. My rule when travelling is to buy brands I can't get at home that are in the same style as I like or from regions I like. Usually that works and I've rarely picked up a dud. Have an awesome time I lived mallorca especially the out of the way places.