Fighting something that has a gazillion hp, immune to everything and kills you when it hits you is frustrating. Tests of patience are rarely fun. Fighting something with a clever AI, even if it has the benefit of prepared positions, ambush etc. is way more fun. You feel like you are fighting your way out of a situation, not getting punished by a designers laziness.
Speaking of AI, the best one I've ever run across was in an old Commodore Amiga shareware game, Mech Warrior, which made excellent use of terrain, weapon choice (range/damage type/heat) etc. often putting up a convincing fight.