Okay. Since Lucky and Tez are being spoilsports (tongue in cheek, guys), I'll go ahead and throw out some of the criteria for "best ever" in my opinion.
Before I do, though, I just want to say that what
Fight Metric is now doing with the major events is AWESOME. While it will always be somewhat subjective, for guys (and gals) who are passionate about the sport, it will take the level of the conversation that much higher.
Now, before anyone gets uptight, this sight is starting with the major promotions, like Strikeforce, UFC, Dream and WEC. It's awesome and just going to get... errr... awesomer as time goes on.
For example. Here's
Ian Freeman. And here's
Frank Mir. If you look at the records and the quality of their opponents, Frank's got a clear edge with regards to impact on the sport. However, I wouldn't put him near the top 10 ever.
Again, this isn't quite ready for prime time, but I'm enjoying watching it grow.
So anyway... getting back to criteria. I'd go with the following (in no particular order):
1: Record
2: Quality of competition (who did they beat?)
3: Impact on the sport (in other words, were they so good at something that they changed the look of the sport).
4: Legacy
So, for example, Royce Gracie might not be able to compete with today's well rounded MMA'ist. But he was a pioneer in the sport, and was integral in shaping it.