In terms of film franchises, the V/H/S franchise is up there. I think every entry has been good so far, assuming you enjoy the horror anthology format.
It's worth thinking about the Dollars Trilogy. Although my understanding is that it wasn't meant to be a trilogy, so I'm not sure if it counts. Maybe it's just three movies that happen to be good.
The Toy Story series isn't my personal favorite, but I expect it would be high on many lists.
The Attack of the Killer Tomatoes movies are an interesting case to consider. It's not safe to assume any are "good". But if you like one, you probably like all of them. See also: Naked Gun.
As for video games, Mario has to be very high on the list of games that are good with very high consistency.