As was seen in my enneagram thread, it took me a while to figure this out (very much in contrast to my MBTI type, which I was certain of after I took my first test [as an adult] and read my first profile), but I'm pretty much certain about it now. I actually received a phone call from my sister when I was a freshman in college, during which she wanted to read a couple profile descriptions to me to see which I thought I was (not completely beknownst to me at the time, she was studying the enneagram, and was trying to type our whole family); she had two she thought I might be in mind, one the more likely, and I'm pretty sure we ended up concluding on me being an 8. I'm still not sure what the other one was (nor does she remember).
Anyway, that, along with some very quick and shallow enneagram study, led me to figuring I was an 8w9 when I first got here. I think my behavior vibed well enough with the 8 (a number of people still think me to be one) that it made sense, and I didn't really feel the need to question it (I was still learning Jung/MBTI, at the time). Then this ENTP I'd never seen post named [MENTION=7421]Bay[/MENTION] said in a thread (started by someone else) about my MBTI type that I was an enneagram 6. I asked him/her what that was about, didn't really get a satisfying answer, so then my curiosity was piqued.
I took a test, got 6, 5, 8 and 3 as my highest fixes, and sx/so for my instinctual variant. I then started a thread about it, and the discussion that took place there wasn't really enough to sway me one way or the other. At the time, I actually started thinking I might be a 5w6, with 6w5 and 8w9 as the other possibilities. Further study made 6 seem really unlikely, as the quick descriptions of the health levels from the Enneagram Institute just didn't fit me at all. And those same studies made 5w6 seem the most probable (the brief health level descriptions just fit).
Eventually, after that thread died and was rebirthed a number of times, and after I'd pretty much exhausted my Jung/MBTI studies, I decided to give the enneagram a deeper look. I bought a number of books (I've now got about six), and, upon diving into the material in the books, I came out believing I'm pretty certainly a 6w5 (5w6 sx/so is still the best alternative, with 3w4 possibly being the next best [although much more unlikely -- it's really just the last fix of my tritype]). Further enneagram studies on the internet confirmed that I was a 6w5 sx/so, and a paid test from enneagram.net also said I was a 6w5, 683 tritype. I've never tested as anything but INTJ, and I've never tested as anything but a 6w5, so I've come to the belief that I just know myself pretty well and am able to understand the tests well enough that, for the most part, when I take a test, it's gunna give me the right result.
I've gotten it down to the point that I'm willing to say I'm a 6w5, 8w7, 3w4 sx/so (the 8-fix could be dual-winged, tho), and, along with being an INTJ, as well as being "Neutral Good" in the alignment system, I feel this does an extremely good job at shining light on who I really am.