Don't forget the actual learning!
Personally, I found aviation exam the most useful as a learning aid, but mainly due to its much better UI - could not stand that orange band across the top of the screen! - and the larger option base. I really liked the fact that you can save a test and come back to it later, something you can't do on BGS or ATPL Online.
I got 95% average (first time passes) earlier this year using AV exam, revision notes and a little bit on ATPL, just to see if there were any major differences. Some of my course mates used ATPL only and got 97/98 all first time passes, and some failed 2 or more exams.
Find whichever one suits you, but don't neglect the books! After all it should be the information that's learned, and not the question bank!