Which is the most forgotten champion in LoL?
On my opinion it's 
Everytime I ask such question, many people say
or
or 
But on my opinion these three champions are not nearly as forgotten as Skarner, and they are remembered for being "forgotten" - as ironic as it sounds. Not to mention, Urgot is already part of meta, now.
However, the same doesn't happen with Skarner, since he is the only one that is literally forgotten. I never see any thread about him on Gameplay & Balance, for example. I have seen very few battles with him, and the first time I saw Skarner being used, it was when he was free week.
Riot itself seems to forget that Skarner exists, since they didn't added the visual indicator that they said they would add to Skarner's passive.

( i dont think I've seen ANYONE use him, even in ARAM)
and even
as well. That's just me though probably.
.
Most of the Champions that everybody posted so far, are indeed somewhat forgettable, but do have some sort of reputation. Talon though, I forget is even in the game, because his design is just so boring and I rarely see anyone play him.