The least amount of cpu would surely be taken by direct passthrough of DTS or AC3, if the audio equipment can process it. And you'll have to use the HDMI audio output for that AFAIK.
Apart from that, I found this
Audio Formats Comparison. I am just a layman in audio formats, but if decoding speed is any indication for less computing power needed, FLAC leads before MPEG-1 Layer-3 (MP3) and Musepack according to the tests listed there.
By the way, why are you asking only about audio formats and not about video formats?
edit: If you're using the 3.5mm audio jack of the Pi, you may also want to test the improved (if experimental) audio driver by adding audio_pwm_mode=2 to your /boot/config.txt. Although it may not lessen the strain on the CPU, it improved the sound quality on my Pi significantly and it got rid of an annoying electric noise in the background. See
http://raspberrypimaker.com/cheap-quality-audio-raspberry-pi/ for an overview of the Pi's internal and optional audio hardware.