I think my absolute forgettable pick is probably the Great Wyrm. I did the Mech of the Week on it (twice, actually) and no other article I've written got so many replies of "I'd forgotten this" or "thanks for introducing it to me." Now, of course, I never forget it since it was a Mongoose mech and I use it fairly regularly, but everyone else does.
But, I think on a more modern mech, there's being forgotten and never being known about. There are plenty of Dark Age mechs that are terrible, but I don't know about them because I never learned. Contrast that with forgetting a mech from the 1980s, which has been in fiction and who's brothers are some of the most famous and remembered mechs in the whole of the pantheon. That would be something.
I'd name the Goliath. One of the unseen, adjacent in the pages of it's TRO to mechs like the Marauder and Battlemaster, and yet if it got 1% of the mentions as either of those two I'd poo my pants. It got a famous mention in the Warrior books, when a battalion of them was blasted to hell, and it's scarcely been heard of since.