According to a poll on lifehacker:
- Firefox 10.4% (1,338 votes)
- Chrome 38.35% (4,935 votes)
- Dolphin HD/Mini 25.29% (3,254 votes)
- Boat Browser/Mini 2.2% (283 votes)
- Opera Mobile/Mini 23.76% (3,057 votes)
For iPhone:
are the same as above plus : Atomic, Mercury, Photon, Switch, Webout, CloudBrowse
In my opinion if you want to be sure you have to test it on different devices, with different screen resolutions, not just on different browsers.