A friend just asked which commentaries he should listen to first. So I suggested:

• Earthshock. This is possibly the best commentary ever! Peter Davison is in fine form, as are his compatriots in arms! Actually, most Davision commentaries are worth a listen.
• The Brain of Morbius The commentators are in fine form in this one, with some real laugh-out-loud moments! This is one of those commentaries where they sound like they're having a great time, almost blokes-down-the-pub in style, unlike some that try to be too technical.
  
• The Five Doctors For those who don't know, the special edition has an easter egg commentary by David Tennant, Phil Collinson and Helen Raynor hiding on the original broadcast version. Unashamed fan-boy alert, and a great laugh.
• Spearhead from Space. If for nothing else, this was the first commentary to feature Nick Courtney!