boomtho: Oh man, we were debating this on the trip back home last night. Let me answer the question slightly differently (at first) - I've been coming to games since I started as a student at 2007, and this is the best home victory I've seen. The other contenders are obviously Tennessee 07, Oregon 08, Arizona 09, and UCLA 12 off the top of my head. This one, however, FEELS bigger - Cal was in a better state for most of those games (UCLA 12 excepted), so we were expected to compete. We knew we had the talent to hang or beat most of those teams, so it wasn't a huge shocker.
This game, at least to me, was a bit shock. This is a program still emerging from the doldrums (albeit with a hell of a foundation vs what used to be there), an unranked team that just lost to a (pretty good) non-P5 team, an unheralded team that just absolutely took it to the #11 team in the country and protected their house. This is a tenure defining win for Sonny that will hopefully reap significant immediate returns on the recruiting trail.
As to where it ranks for overall Cal program wins? I tend to give more weight to happen in seasons where you're more of a realistic contender, so I'd still go Tennessee 07 and Oregon 08 as bigger wins.
But this may have been more satisfying.
LeonPowe: Since Sonny has been hired. Everyone recognizes this is a flawed team, but I've seen people talking about that our defense has regressed to 2013 standards - and I'm just not seeing it. Yes, we're not good, but I think we have some kids that play hard, and unlike 2013, there is some foundation of talent there - not walk-on wide receivers. Our run defense could use help, but there was improvement from SDSU to this week (just see the special teams!).
Piotr Le: Probably since we last beat a Top 25 team in 2012 against UCLA. I think it is because it was a game that had to be had, to upset expectations and the National Texas hype train.
ragnarok: boomtho, I like your list of contenders, though I'll add a couple of road games to your list: Oregon 07 and Stanfurd 09 (Texas 15 also makes the list, but not at the top). My answer is that UCLA 12 might be bigger, and Furd 09 definitely was (still the last time we've won the Axe...), but we probably won't say for certain until we know how good this Texas team really is later in the season.
Ruey Yen: I am probably in the minority here by saying that this win over Texas is merely the biggest Cal win for the program since the win over Texas exactly an year ago in Austin. These two victories over Texas are no doubts the biggest wins in the Sonny Dykes era (hot take - if Sonny Dykes only views the UCLA, USC games as highly as the Texas games).
Sure, the Longhorns already had a loss last year and came in with a lofty over rank of being 11th this time. Nevertheless, it is a similar squad with plenty of athletic talent and some flaws. With this loss, they could easily spiral into disarray and this win may not look all that great by the end of the season.
Now, if we change the question to "the biggest Cal HOME win" - I would say this was the biggest win since the Tennessee 2007 game. You know, this one.
Nik Jam: I haven't been a Cal fan that long, maybe 10 years. This might be the first win that completely changes the outlook of a season from "for sure a losing season" to "The Sky's the Limit".
Arizona in 2009 changed the complexion of that season from an '07 like collapse to a respectable season, and gave the confidence the team needed to win The Big Game. So that is the closest in my experience.