Considering they pretty much all got movies/show appearances its tricky to say. The one I forget the most are probably phione and genesect. The first is generally forgettable, its gimmick being that its like a normal, evolving Pokémon and the second has always felt off for Pokémon for me? Cool design just odd.
Deoxys (Regular Form). I mean, c'mon. When you think about Deoxys you think about it's three forms, not it's neutral form.
Not counting Gen 9 or the Pokémon others have mentioned I think it’s Regidrago. The original Regis have the nostalgia factor, Regigigas will always be remembered for its awful battling prowess, and Regieleki is fantastic in competitive battles. Regidrago is just average in battle and you rarely hear people mention it. It’s a shame since I actually like this Pokémon.
Marshadow and... that one electric mythical I can never remember the name of (apparently its Zeroara). I always forget those two exist unless I'm looking at a list of mythical Pokémon.
Probably Eternatus. I mean, when you think of Legendary Pokémon you think of Pokémon like Groudon, Mewtwo, Ect. not eternatus.
I agree with the one above. When thinking of Legendary Pokémon, I was gonna say Heatran but Eternatus never even crossed my mind. Hes supposed to be a 'big bad' legendary and almost part of a trio, but aside from iyts freaky design, it feels kinda forgettable?
I'd have to go with Heatran, actually. I mean, what even is its purpose? If I remember correctly, it is supposed to watch over/control volcanic activity, but that may just as well fall to Groudon, in my opinion...
Can I just say I did not remember Heatran at all before reading this thread? Also I'd reckon Cresselia; mostly because everyone and their mother probably think it's a mythical Pokémon (when according to Bulbapedia it's actually a legendary)