A few candidates.
Deadlock - Hybris - dunno if they're done, it's only been six years, but it's the only album without Sabine at least doing guest vocals, so that automatically knocks it down. Margie was only supposed to be a temporary replacement, so it'll depend on if they make another album (it was a bit auspicious that Sabine and Sebastian did an acoustic number relatively recently, but who knows).
Imogen Heap - Sparks - I kind of doubt she's done, but it's been eight years, and she seems to have given up on the album format for the moment. I want to revisit this one sometime again, since after a few listens when it came out I was massively disappointed. But as it stands now, it's a whimper compared to her first three.
Ram-Zet - Freaks in Wonderland - it's been over a decade now. This album has grown on me in recent years, but it's still probably their weakest, though still quite good.
Skyharbor - Sunshine Dust - it's only been four years, so I don't know if they're done, but unless something surprising happens and they get Tompkins or someone even better, this is going to remain their weakest for now.
Vanessa Carlton - Love is an Art - also probably unlikely she's done, but I just haven't been able to connect with this album yet like all the others.
Vertical Horizon - The Lost Mile - dunno what Matt was thinking, this was not a direction I cared for him taking, but again, hopefully it's not the last.