Alright, just so you have an idea of where I'm coming from; I studied French and German GCSE. Since then I've travelled around a lot, and have picked up Spanish to a level where I can read most of a menu, and can politely ask for a beer (but not much else) in 7 other languages.
Everyone who's said German is easiest (for an English speaker), they're right- gramatically it's one of the closest to English, and is also very consistent in it's grammer, verb conjugation, etc.
If you've studied French and Spanish, Italian should come pretty easily- a lot of the words look the same and the grammer is simliar to ther western European languages, as it has a simliar Latin grounding
The Scandinavian languages are similar to German, and a bit further removed from English, and as someone said, are all very closely related to eachother (for instance Norwegian only became a formal language after they got independance from Denmark in the 19th century).
The various Slavic languages (Polish, Slovacian, Slovenian, Albanian, etc.) are less like English, and are all broadly similar through common ancestry, so learning one can provide a spring board into others.
But, if you want to learn something difficult and completely unrelated to any other language on Earth, try Hungarian, or perhaps Basque. Both are unique.
Hope this helps you out some.