Mobile phones are becoming more and more powerful, therefore an Android ps2 emulator was a matter of when not if. Playstation 2 android project, PTWOE, is the pioneer among mobile emulators, already being able to run games at playable speeds, download the newest version and see for yourself. It can rival the PSP (ppsspp) and Playstation Vita (AN.VITA) emulators. Please keep in mind it only opens iso files right now, make sure you download iso versions of games.
Please keep in mind that while the experimental version is faster, it also lacks in compatibility, and it is not as stable. If you are experiencing issues with it, please revert to stable version. Some devices have trouble detecting bios, depending on the version.