I see that we have already got a clear leader. Let me explain for Android users what "Access to 3000+ questions from GMAT Club forum" means and why it's a very useful tool....
When it appeared in GMAT ToolKit for iOS for the first time, it was called "autoscan". The app scans GMAT Club forum, finds, separates questions posted by our members, formats them, and adds to the app as a separate question set. Although those questions do not contain the official explanations and you can only practice PS, DS, SC, and CR, you can get a lot (3000+) of really good questions with direct references to the forum. Often explanations by our members like Bunuel are even better and more valuable than the official ones. So, the app will never run out of questions as a set of 4000 questions (including almost 1000 built-in questions from our partners) is big enough for any preparation. It's easy to keep all your history at hand and practice those questions offline.
Honestly, if I were you, that would be my first choice too....
_________________
iOS/Android: GMAT ToolKit - The bestselling GMAT prep app | GMAT Club (free) | PrepGame | GRE ToolKit | LSAT ToolKit
PROMO: Are you an exiting GMAT ToolKit (iOS) user? Get GMAT ToolKit 2 (iOS) for only $0.99 (read more)
Math: GMAT Math Book ||| General: GMATTimer ||| Chicago Booth: Slide Presentation
The People Who Are Crazy Enough to Think They Can Change the World, Are the Ones Who Do.