I think it's in the eye of the beholder. No two people or groups are gonna come up with the same list.
If you just want stats, it's easy to find the list of schools with the most wins. Or most claims to national titles. Or more bowl wins. Or most Heismans. Those stats should certainly all be part of the equation. But not all of it.
I think history and tradition and compelling storylines and conferences are all a part of it too. And the impact and contributions of the greatest coaches for each school, as well.
In short, it's gotta be a holistic thing. Which makes it really complicated. Which is why no two people will come up with the same "formula." There's really no exact right answer.
But there are wrong answers: like including Florida State in the Top 15 of an "All Time" list, when they didn't play football for more than half the game's history, and didn't become nationally significant until less than 40 years ago.
Then again, some people put these lists together weighting what happened recently much more heavily than what came before. I don't like doing it that way, but ESPN seem to love doing so.
Should Ga Tech be Top 25? Without carefully thinking through all the other teams and their relative merits, I'd say probably, yes. But I reserve the right to say I was wrong, later, because there are a LOT of great programs out there.
Go Vols!