I really like ExChess for its flawless compatibility with Chess for Android. It can use multi threads and delivers one of the best chess performance among the Xboard/Winboard engines. At present only Crafty is playing at relatively "close" ELO level.
The new version 7.88 beta is now ported to Android by Jim Ablett. I expect this one will break the 2900 ELO barrier when i'll have time to introduce it in Rapidroid.
The package below includes both arm5 and arm7 versions and a book as well, in case you want to use it. Don't forget to install search.par too before running the engine. As far as i remember the book is not crucial but the search.par is absolutely necesssary.