anything that gets you moving and into the right BPM zone for your age :]
its good to vary exercises, so instead of say 45 mins just on xtrainer, do 15 mins on treadmill, 15 on xtrainer, 5 on rower (can do some 100/200 slow/fast splits, should get 1400m+) and 10 on bike.
this stops you getting so bored ;]