First, some oldie moldies, E.E. "Doc" (the "Doc" is compulsory

) Smith's series, all of them, the Skylark series, the Lensman series and, for completists, the Subspace Engineers series. Doc Smith proves that in SF nothing ever dies, no matter how bad, because Nostalgia Rulz!
Spenser, you say? Please, that's one of the best lasting series detectives written.
All mystery series detectives see their better days soon behind them. Most indeed only have one really good novel in them. Even Sherlock Holmes suffered from open ended serialization. Would that most series detectives were still as readable as Spenser. Or Aurelio Zen, who's made a comeback (since Michael Dibdin's estate was probated?) Even Zen should have stopped.
Going back to SF, Dragonriders of Pern wasn't so good I thought and every novel thereafter should have been aborted.
Even more controversially, the Foundation Trilogy should've been.
I will second Dune, but add that it's not just Brian and Kevin J. Anderson at fault. Frank took that sucker to the well too often on his own.
The Riverworld series should have been dammed.
I can't say from my own reading but do fans of Captain Future and Perry Rhodan really like the later novels/stories as well?
A. Bertram Chandler's Commodore Grimes and E.C. Tubbs Dumarest series were barely readable and bloating them hurt even more.
I couldn't even read Dan Simmons' Hyperion series.
Actually, the only SF series I know of
without rumbles from the fans are C.J. Cherryh's Downbelow series. Which I am not going to start because it's too damn long and the success rate in series is so damned low. Still, it would be interesting to know what fans here think of it (if there are any.)