Pulse code modulation, go for DD or DTS whenever you can unless your receiver can decode multi channel PCM which would be better. Dolby True HD and DTS HD are uncompressed formats along with PCM. My receiver only decodes PCM in stereo :/ but does the other stuff just fine.
Dolby Digital and DTS are the compressed (sort of in the case of DTS) multi-channel formats on most blu-rays and DVDs. PCM is the uncompressed "stream" that the player can create from the Blu-ray, for feeding into AVR receivers that might not support DTS or DD - Although 99.9% of current units do.
PCM also allows secondary audio - like directors commentaries and that sort of thing to be mixed into the stream.
There is an argument to suggest that different players and receivers might make small differences to the decoded stream, so you might possibly hear a difference.
There are also limitations as to the number of channels that can be sent via the PCM stream, as well as the maximum bit rate and sample frequency. For all these reasons, the general advice is allow the receiver to do the decoding from the HDMI input, with the player switched to DD/DTS