Walgreen's discount color film.
Currently Walgreens uses Fuji as their supplier. I don't believe I've shot any of the current Walgreens film, but if it's anything like other store-brand Fuji I've shot recently, it's about like any other Fuji consumer-grade film -- far from the worst in most respects.
For at least a decade prior to Agfa's demise, Walgreens used Agfa as its supplier. This film was just like other Agfa consumer-grade films, and IMHO it wasn't bad at all. I happen to like Agfa's overall look.
I don't know who Walgreens used as a supplier prior to the mid-1990s -- I don't recall buying any Walgreens-brand film prior to 1994 or 1995. It's conceivable they used something subpar before then.
More generally, though, I'd say that there really is no "worst film." Even stuff that's dreadful by most measures (grainy, bad color for color film, etc.) can be good for specific purposes. I like a grainy look for certain B&W subjects, such as old buildings. Poor anti-halation characteristics are generally considered bad, but they can work for certain subjects. I've got a photo on my wall, shot with Fomapan 400, of the interior (as seen through a door) of a fire-gutted mill. A hole in the ceiling has a halo around it because of the film's poor anti-halation qualities, and IMHO that halo makes the photo -- it draws attention and speaks to me, subjectively, of the renewal that will inevitably follow the fire. (A new middle school is now almost finished on that site.)