I'd vote for German - it's what I'm learning myself.
Although Spanish is the second most widely spoken language worldwide, Germany has the most native speakers in Europe and is the second most spoken language in Europe (after English). Let's face it, if you go to Majorca they all speak German!
German turned out to be harder than I expected, and it's somehow not as easy to skim read a newspaper article and understand what's going on as it is in French or Spanish (more words seem to look similar to English words in those languages). However, as Andy in Sig said, now I know German I seem to be able to understand written Dutch (can't make head nor tail of spoken Dutch) and it makes reading old English like Chaucer easier too.
Plus, the Germans are nice, tidy people whose trains run on time and make fantastic cakes.