I'd go with the Spanish, for reasons mentioned by others...it's relatively easy compared to Fr or Ger in terms of grammar, widely-spoken in world terms and also easier than Fr Ger It or esp. Port. in terms of pronunciation; it is almost entirely phonetic, meaning that once you know what sound a letter (or pair of letters) makes, it remains constant. Italian is phonetic too, but has more inconsistencies. Portuguese is difficult in terms of pron., but is a superbly sexy language (and a splendid country!) However, knowing Sp. will give you a fantastic headstart in both Port and Italian, as they are so similar. Also, ahving some Fr will help your Sp vocabulary development. I speak v. good Sp, and reasonable Portuguese and French. On a recent trip to Italy (my first) I was easily able to communicate, and impress/befriend the natives.
Remember though that a language is not just for Christmas, it takes time, and the more you devote to it, the more you'll get out of it. The good old BBC has some excellent online language learning stuff here:
http://www.bbc.co.uk/languages/
try some out. Good luck... knowing other languages opens doors and your mind, too.