For sheer size and scope, the PSU tailgate scene is hard to match. Never have I seen as many tailgates (many of them very elaborate and well equipped) as I saw at Penn State. The simple fact that Beaver Stadium is largely surrounded by open fields allows for that. That, and the fact that PSU draws extremely large crowds.
Tailgating at stadiums that are surrounded by campus buildings, apartment buildings, or other residential structures necessarily means that tailgating is gonna be smaller in scope. Cal's Memorial Stadium is like that. At Cal, fraternities all host pregame tailgaters for members (both undergraduate and alumni). I think the sororities and cooperatives do the same thing. Fans also park at various locations in and around the campus, and tailgate at those locations. It is completely decentralized, given that there are no parking areas or open fields immediately adjacent to the stadium. Just prior to game time, people pour in from a multitude of different directions. My fraternity house is one block from Memorial Stadium, so it is a convenient place to park and party, both pregame and postgame. I don't really care about the Tier ranking. I am more inclined to regard the tailgate experiences at PSU and Cal as simply being different, rather than one being "better" or "worse." Of course, if size is the metric, there is no comparison. (Cue comments about size mattering ...)
USC's taligate scene is kind of in the middle of those two extremes. there are some open grassy areas surrounding LA Memorial Coliseum, and people tailgate there, but the open acreage is FAR less than what PSU has. Stanford has a bunch of asphalt and dirt parking lots around its stadium so it can accommodate numerous tailgates, but the 'Furds draw very poorly, so the tailgate scene there is typically underwhelming to say the least. UCLA plays at the Rose Bowl, where there is ample room for tailgates. When the actual Rose Bowl game is going on, they park cars on Brookside Golf Course (next door), and the grass and landscaping make for a good tailgate environment.
I've always heard that the tailgate scene at the Grove (Ole Miss) and Neyland Stadium (Tennessee) are epic, so I was not surprised to see them listed in Tier 1. I hope to take in each of them in the near future.