Okay, I'm a "old" guy and I've chased some unusual events living in New Mexico.
My first tornado was May 30, 1982 near Mountain View, Oklahoma. That's how long I've been doing this.
My greatest regret --- April 26, 1991... I had a class that week and could not get away for Red Rock and the others that day. Just part of life... I pleaded with my boss, but I knew it was useless. Jobs are more important than storms...
My greatest event was actually a shocker --- August 31, 2008 just west of Las Cruces, New Mexico... Two supercells that produced cyclic tornadoes for 90 minutes in the summer monsoon. It happens.. I still can't get a good count of how many tornadoes there were. I've settled on eight but it could be 11. It went on and on like a machine, although much of it occurred after dark. I learned a lot about vorticity that night.
Otherwise, in the Plains... Too many to really choose from, but I consider June 6, 1989 near Plainview, Texas to be one of my greats. I learned so much that day. It was not anything more than an F2-F3 tornado but that day I learned how to chase storms.
"Greatest" and "best" is different to everyone. To me, the greatest is when you see something different and learn something new. So that's why I'd pick August 31, 2008 in the monsoon and June 6 at Plainview as the greatest in my career. Hopefully there will be many more.... As some say, size does not matter!!