Any engine now play at least on the level of 1900, so they will be more than a challenge for most players.
I suggest using the engine, which plays in your preferred style. For example, I use Rybka Dynamic, as it often goes for open agressive positions, I like. It's also easily gives away material for tactical advantage.
Personally I hate Fritz. I played it, and it's totally opposite. Its slow and methodical. Any human would go nutz, playing vs a computer with a strong defensive mindset.
It also depends on the opening book choice of course.
I'm using arena a free chess program.
Like many other chess programs it can use open-source chess engines
It plays to strong for me, as I'm a beginner what engine would can you recommend me. I'm now at ~ 1050 (climbing slowly) rated here. I'd like to win sometimes too.