Well, first off, let's be clear on what the word "definitive" means:
http://dictionary.reference.com/browse/definitive
–adjective
1. most reliable or complete, as of a text, author, criticism, study, or the like: the definitive biography of Andrew Jackson.
2. serving to define, fix, or specify definitely: to clarify with a definitive statement.
3. having its fixed and final form; providing a solution or final answer; satisfying all criteria: the definitive treatment for an infection; a definitive answer to a dilemma.
So really it has nothing to do with personal preference at all. Whether a version is your favorite and whether it's definitive are two separate questions.
The theatrical edition is literally an unfinished film. They didn't meet the rigid deadline for release and were thus forced to send out what was essentially a workprint, a rough edit that hadn't yet been trimmed for pacing, was still missing some of its effects shots, and had only a crude temporary sound mix. Wise hoped to be allowed to finish the film and have the rough print replaced a few weeks on by the proper, finished version, but he was never given the go-ahead to do this. So the theatrical edition is not the most complete or reliable version, nor is it the fixed and final form.
And the ABC "Special Longer Version" is just the unfinished theatrical release with a bunch of deleted footage shoved back in, including material from the aborted "Memory Wall" sequence that has Kirk and Spock in the wrong spacesuits and includes unfinished FX shots like one where you can literally see the wooden scaffolding holding up the outer-hull mockup. Again, not complete, reliable, or final.
So that pretty much makes the DE the definitive version by elimination. The only definition that would even potentially allow either of the other versions to be called "definitive" is number 2. The version that most people think of when they think of the film could be considered the definitive one, the one that defines TMP in their minds. That would most likely be the "Special Longer Version," since it's the one that was just about exclusively shown on television and released on video for a good two decades, and is thus probably the most widely seen version, at least prior to the DVD releases of the DE and theatrical cut. Still, I resist the notion that the highly flawed TV cut deserves the sobriquet "definitive."