Heavy assault gets a vote from me, too. Defense missions leave you worrying too much about whatever ship you have to babysit for you to actually have any fun. Intercept missions are fast-paced, but much like pure defense, you spend too much time shooting down bombs and not dogfighting. Bombing runs can be fun, but bombers themselves are generally sluggish, making it abnormally hard to avoid your target's defenses, or enemy interceptors.
For good examples of classic heavy assault, check out Hellfire and Secrets Revealed (sort of) (Silent Threat), A Lion at the Door and Into the Lion's Den (FS2), and Small Deadly Space, The Darkness and the Light, La Ruota Della Fortuna, and Reaching the Zenith (FS1).