This is a difficult one for me. For the most part I tend to evaluate artists on peaks rather than consistency and longevity, so often I'm only coming back to a small handful of albums by most artists anyway (and reasonably often it's just favorite songs). So I think it's more a matter of what artists dipped so far that it actually sullies my feelings about even their best albums, and really the only good candidate for that category for me is In Flames. Either grouping of The Jester Race-Clayman in particular is incredible, but their work in the 2010s was quite frustrating.
So I guess the best way I've thought about to view the question is what artists do I like that don't really have one particularly outstanding album or two, it's mostly that I like their output as a whole and a lot of different songs scattered throughout their discography, and if you chopped all but three out they might not have enough leftover to compete quite as well with more concentrated favorites. So that leaves me with some candidates like Devin Townsend, Rush, Delain, Miles Davis, Bob Marley, Autumn's Grey Solace, Emily Remler, King Crimson, Arch Enemy, Genesis, Dark Tranquillity, Sara Gazarek, Perturbator, Yes, Carlos Santana, Novembers Doom, Wayne Shorter, Entrails, Thelonious Monk, Skinny Puppy, the Scorpions, Siouxsie & The Banshees, Stevie Ray Vaughan & Double Trouble, Frontline Assembly, McCoy Tyner, Edge of Sanity, Tommy Flanagan, Swallow The Sun, and Oregon. Still love all these, but I'd probably think slightly less of them if their discographies were chopped down to just three.