The answer is really pretty easy. There isn't a definitive Fallout game because there isn't a definitive Fallout fan. It will be great fun arguing about it, though.
Let me begin by utterly failing to impress you with my Fallout pedigree. I wasn't yet a teenager when I discovered Wasteland. That game, along with F1, F2, and F:T is still on my hard drive. I've walked the wasteland playing the same character for twenty years now. For me, the definitive Fallout game started with Wasteland - even though when I boot up today I am playing F3 instead of Wasteland. It is hard to capture the feeling of discovery that playing that game gave me.
And you can't go back. Even though I'll still play all of those games, it isn't quite the same. Nostalgia is a powerful force that defies all reality. Ask your grandmother why music, politics, education, and general life svcks more now than it did in the Great Depression and you'll get an idea of what you're dealing with.
I think that F3 is a spiritual descendant to be proud of. (Kind of the same way that BOS was not.) The post-apocalyptic genre was just about dead for a long, long time. I intentionally purchased crap games that fell into that genre just in the hopes that someone, somewhere would see it make enough money to publish another game instead of Ultimate Madden Football XXVIII. The greatest gift that F3 will give us - whether you love it as a work of Fallout genius or hate it as eye-candy tripe - is that it showed that there is still a market for this kind of game. The better F3 does, the less likely we'll have to wait another decade for F4.
Personally, I love this game. It is less campy than F2, and campy was fun . . . but the first person POV makes for a more immersive experience. I never felt the adrenaline thrill from just peeking around the corner in any of the precursor games. In the past games, I've never been crouched down while two raiders loomed over me - low on life and down to 2 shotgun shells - only to pull out of it with two head-shot criticals through VATS. Yes, the characters could use a little more development, the dialogue could be polished some, the skill/stat system was overly simplified, and god help me but they should've made the radio stations where you can load in your own tunes. . . but overall, this game is what I hoped it would be and nothing at all what I was afraid it would turn into. It isn't perfect, but by god the mods will come and that is something that none of the other games had. There is a chance for the community to turn this into their own definitive Fallout experience and I get to cherry-pick the best among them.
My favorite part of every Fallout game was being down to my last handful of bullets, no stims, and praying that Dogmeat won't die. F3 has given me that - even if it didn't last past 10th level.
Definitive? No - unless this is your first journey into the wasteland. If so, then welcome to the madness and I'm sorry because it'll never be as good as this no matter how great the next one is.