DragonKing: GnuBg is what I use. It's the strongest in the world per $, lol. It also beats almost everything that you'd pay $ for.

Set evaluation and analysis to 2-ply (ie. Supremo) and it'll give you a good fight.

Small tip. Whenever you change any settings in the program, they are only temporary changes unless you use the Settings/Save Settings menu option. It's a pain in the bum when you forget, especially if you've been creating a beautiful 3D board setup and exit without saving. :-/