Hands down. His games were brutal and challenging.
Often times, we'd bring misery upon ourselves because we weren't paying attention. It'd get late in the session, or some of us would have come off a hard week of school or work and were just tired.
For example, when we heard about the "biggest owlbear ever!", we decided to go look for it in the hidden valley where it supposedly lived. It was a "side trek" which had little importance to the main plot. We figured, why not? Let's go kill an owlbear. But we didn't expect an owlbear the size of King Kong...
Another time, after exploring the Sunless Citadel, we returned to town to inform a shop keeper than her son and daughter had died in the dungeon. For whatever reason, we decided to send in the group's half-orc to tell her this sorry news. The half-orc need to buy some equipment anyway. Little did we know was that the shop keeper was racist against half-orcs, and tried to price gouge him. The half orc got mad. told her the news, she screamed, he drew his weapon, and next thing you know we're kicked out of town!
To this day, I'm not sure about the reasoning behind sending it the half-orc.
(I'm not even going to relate what happened to Meepo, the kobold.)
See, in Jim's games, you had to pay attention. There wasn't much wiggle room for failure. In combat, you had to use teamwork to get every bonus available. Jim wasn't going to pull punches. It amazes me we didn't have more TPKs, given some of the mistake we've made. Characters did die, but it was usually a character who's player said the infamous: "Come'on guys! They're only 1st level fighters!"
Thanks for the great times, Jim!