The base seniority varies by seat and month to month. Here's my take on the list from senior to junior.
DAL, super senior on the Capts side. On the FO side I've seen it take over a year to get there and also seen guys sent there who didn't even want it.
HOU, similar to DAL, but a bigger base, and not quite as senior on the Captains side but about identical on the FO side. A lot of middle seniority HOU guys commute from DAL.
PHX, Moderately senior on the CA side. Large base and should have higher retirement rates in the next few years. On the FO side, guys who want it seem to get it within a few months.
MCO, OAK, MDW, BWI are all about the same as they trade off who is the most junior in a given month. Bases usually seniorize over time since open, but MCO as a new base quickly jumped ahead of the others on the Captain side.
According to the seniority calculator on the swapa website, As a 15 year person I would be in the top 10% in BWI, the bottom 8% in Dallas, and right smack middle of the pack in PHX. I would be the almost exactly the same top 20ish % in OAK, MDW, and MCO.
Maybe an FO can tell you what it says for them.