The most succesful member of the Big Bang Theory cast?

It depends on what you mean by "Most Successful".

If you mean, "Big Bang Theory moving forward", that's one thing, but if you take pre-BBT careers, maybe not. Also: are you including non-acting/entertainment industry stuff?

Johnny Galecki was not only part of one of the most popular sitcom of recent years (BBT), but also a big part of Roseanne, which was commercially and critically very well-regarded. That has lead to spinoffs in recent years, too. He's been in films, but never as a major role in a big-deal film (to my knowledge).

Mayim Bialik was part of BBT, but also has a PhD in neuroscience. So, that speaks to success in a very different realm than on TV.

I'd argue for one of those two, personally.

that's not saying much

In terms of money wouldn't that be Jim Parsons?

In terms of current career peak wouldn't that be Jim Parsons?

As someone else asked how are you defining success?

I would say Jim Parsons is the most successful. He currently does the narration on Young Sheldon and was nominated for a Golden Globe and an Emmy for the show Hollywood. He was also in the films The Boys in the Band, Extremely Wicked, Shockingly Evil and Vile, A Kid Like Jake, Hidden Figures, Home and The Normal Heart.

Yes,100%! Jim is close second.

I haven't seen her in anything since BBT but commercials. It's like she's now living Penny's career.

She worked with Captain Kirk.

Nuff said.