Tarazuma said:90% of Gen 4 is forgotten to me, seriously, I can't remember that junk. Some of Gen 3 slips my mind as well..
I don't know if there's one single Pokemon that's overly forgotten, but I know Hypno is the most neglected. He only had like 3 minor anime appearances and never had any unique toys or plushies or anything (was just included in toy series that include EVERY Pokemon).
Even in games he's neglected. In Pokemon Stadium, there was a minigame with 4 DROWZEE'S and a PENDULUM. Made no sense, why wasn't it 4 Hypno's?
And worst of all, in Pokemon Rumble, Hypno is the ONLY fully evolved Pokemon that only has a Rank C form, the weakest rank in the game. Every single other evolved Pokemon has at LEAST a Rank B, (or Another Rank B, A or S) It's true too, I checked on Serebii's Pokemon Rumble page. Honestly, there's really no excuse for that other then Nintendo/Gamefreak downright HATES Hypno.
Psh, don't get me started at how Nintendo neglects Waluigi. I will go OFF.
Anyways, the most forgotten Pokemon in my opinion would be Dunsparce. You could call it the most popular forgotten Pokemon (if that even makes sense xD).