For fun I like boxing/mma because it has other benefits within the cardio. Also, soccer, mountain
Biking etc.

With a specific goal such as bulking I think swimming would be good because of limited joint
stress. Now I hate swimming and don't do it, but it still applies. Low intensity walking/jogging/eliptical
As well.

Cutting : I like a mix of aneoribic and aerobic. Boxing circuit followed by two mile run. I did
Randy Couture's complex several years back which is a barbell complex. 0lympic bar with 25's
(Or whatever) 7 different exercises for 7 reps before putting the bar down and resting for a
minute then another rotation for 7 cycles as I remember. Hard as hell!