Those are a couple different questions you're asking here.
The single target platform with the most users is the PC. Even counting just gamers, the biggest MMO alone (Happy Farm) had more than 200 million players at its peak. Compare that to some 70 million sold iPhones (all models, as of end of 2010), 190 million sold Android devices (as of end of October 2011), XBox 360 (55 million units sold), Wii (almost 90 million units sold) or PS3 (50 million units sold) - and only a portion of iPhones and Android devices are used for gaming.
The platform which is the best for you to get the most users depends heavily on what style of game you want to do - and on your marketing budget.