It's hard to pinpoint which one, because everybody have a first/favourite gen bias that makes us say
>they're all memorable,
So here's a list of the most forgettable for each gen:
Kanto/Johto
Agatha pretty much is known only due to gen 1 pandering and appearing in the anime, but she's easily forgettable due to a relatively lacklustre design. But the most forgettable Kanto E4 is obviously Koga, due to appearing way too many time as a gym leader (and getting overshadowed by his own daughter). Close second is Will, since Koga is remembered (for the wrong reason but still)
Hoenn
Despite being the first, which usually makes them memorable, Sydney is forgotten quite often (which is also sad, since he has a kickass design) . Glacia is also quite forgettable, tho she does the whole "old but strong experienced trainer" shtick much better than Agatha.
Bertha suffers from the same curse of Glacia, and is made worse by the fact that she's third old lady E4. Lucien is a strange case, he is forgettable, but more than one of my friend thought that him and Will were actually the same person, which admittedly would have been cool.
Unova
Let's be honest, they're the most forgettable. All of them. I'll probably be shunned because of this but it's objectively speaking the truth. In a plot heavy game the fact that even the gym leaders are far more plot relevant than them, that all of them, with the whole Psychic/Dark/Fighting/Ghost shtick, seems more of a bad fangame attempt at making a new E4 won't help in making you remember them. Yeah yeah, Caitlin appeared in the BF, but that is usually unknown to the most, or usually forgotten. Also, yes, Shauntal filled briefly the big tiddied goth gf quota in porn but was rather quickly replaced by Hex Maniac, which just show how forgettable her design really is.
