2004 hands down.
In the spring of 2003, I was resolved to chase every big setup that came to the Plains. And mostly, I did. 14,000 miles, four round trips to the Great Plains from WV. I saw no tornadoes. I got one or two lightning slides and only a few minutes of usable video. The reason? No room for error in 2003. I was so close several times, but one mistake in 2003, and the action is suddenly 100 miles away from you and no catching up was possible. The one perfect chase day was of course June 24, and I would have jumped on the road for that one too in a heartbeat if I'd seen it coming even slightly. I had the means and the resolve, but once it was evident it was going to be a good setup, it was too late to make it to SD.
2003 also didn't hand out consolation prizes to the busting chaser (lightning, storm structure). In 2003, when you went home empty-handed, you went home *empty handed*. By contrast, 2001 was a year when I saw no tornadoes, but consider one of my best chase experiences ever. 2001 was filled with so many other good catches that the lack of tornadoes almost went unnoticed. 2004 gave out tornadoes on the good days and consolation prizes on the bad ones - for instance, June 10 and May 23 didn't give me tornadoes but put on some memorable shows to take back home. But in 2003, I came home from the Plains four times wondering why I went - I had nearly nothing to show for all my effort.
2004 is a year that proves that chasing is all about keeping at it. The good years will come only through the bad ones. And it's worth it!