Whatever he is, he is surely not “forgettable.”
Harry Hamlin. Not a high impact guy but keeps busy.
Shatner maybe isn’t a, er, stellar actor, but he sure is a remarkable singer:
The entire generation of late-nineties teen actors: Freddie Prinze Jr., Matthew Lillard, Ethan Embry, Tara Reid, the entire casts of the “American Pie” movies, etc. It was a bumper crop of uniquely forgettable, uninteresting, no-talent bohacks that had a surprisingly long tail.
I’d have to disagree with you on old Billy boy. 
Ever since Aliens I always notice him in movies. In fact I’ve got the reverse problem with him, any movie I see with him in it, I immediately recognise him as Bill Paxton, rather than whatever character he is supposed to be playing. :smack:
True. :o
Unfortunately for me, I have made it a point to know the names of almost every actor in every movie I ever saw! Especially the old B Westerns from my childhood. I’ve known a few other people (mostly fellow “actors”) who have that knack or passion or affliction for knowing the names and bio details of screen people. It’s even a fun game to play with those folks to try to stump each other with the most unlikely player to be known by his/her real name. (There’s an even more fun wrinkle to explore: REAL name, and not just SCREEN Name. – Like Issur Danielovitch!)
So it’s a tough challenge for me to name somebody I was never impressed with.
How about Frank Thring (I) (1926–1994) whose claim to a slice of my memory would have been his role in The Vikings (1958)
Eric Roberts is also forgettable in most roles, but is one of the busiest DVD-movie guys out there.
Being a miserable old crank, I would say MOST of today’s crop are unremarkable in looks or talent. They are successful because they are in whatever is put out there for the public. Jennifers and Jessicas and Kates, all get mixed up in my feeble mind, as do all those musclebound bald action heroes and men in tights. Ben Affleck, Jennifer Lopez, Jennifer Anniston, Natalie Portman…I realize I’m picking on the women here too much, but the men, I hardly remember their names. They are all OK, I guess. I just don’t get the enthusiastic groupies who rave over them. But, that’s the way it goes. After all, QVC and HSN make a mint selling cheap blue topaz - ‘look at that Londonderry Blue from our exclusive mine! For Valentines Day, anniversary, birthday, mothers day - she will love to get this fabulous ring.’ Yawn. Ryan Gosling, Bradley Cooper…OMG I forgot the KING: Channing Tatum! BLAH!
Exactly, forgettable means “I can list three popular movies he’s starred in and you’ll still have to go to IMDB to fine out his name.”
Steve Guttenberg.
One other detail: the term “successful” may be in doubt unless you count movies and shows where this performer appeared before the show was cancelled or the movie failed to return on its investment.
I give you show-killer extraordinare Dylan McDermott whose very name in the credits is a signal to try something else.
I endorse this post! Seagall’s dreadful, but he’s not forgettable. Keanu Reaves has a very narrow acting range, but he’s not forgettable. (And he was perfect for the Matrix.) I’m talking about people that fade into the scenery after the hype disappears, because they’re kinda like any other blonde/brunette/black-haired actor or actress.
I guess I should have defined “successful” more carefully … 
Ah, but remember him in Aliens? That was unforgettable, maybe even stole the flick.
I saw Sonny Tufts on “My Mother, the Car,” so that should give you an idea how forgettable he was.
Steve Guttenberg isn’t forgettable. My first thought was “Three Men and a Baby.” My next thought was “Police Academy.” Sure, he’s not a leading man type like Ted Danson or Tom Selleck, but most people could name him.
Good example. Looking at his face, I remember his face, but looking at his name, I got nothing. Still couldn’t tell you what he’s been in without checking the IMDB.
10-4! I had to scratch my head big time before deciding he’s the one I meant. I was hung up on Dermot Mulroney for some reason and this guy is pretty reliable as an actor.
ETA: Look up Fay Spain and see how many others she reminds you of and if you can name any of them!
I’m a miserable old crank with you, then, because I came to say the same thing. Many of them have nothing much going beyond looks and the ability to interview well.