I'd agree with the many sage comments here about not mistaking 'interpretation' with acting, so no problem with Beltran or Blalock's performances in their respective roles. I know people who dislike Combs, Alaimo, Robinson as 'showboaters', and I have my own issues with Brooks and Stewart declamatory style.
Strangely, I think Frakes and Sirtis would have been better with more comedic parts. Their commentary on Insurrection is one of the funniest ever, and though I think Sirtis lousy on TNG, her best scene was that bar one in FC.
For overall worst regular though, my award goes to McFadden. It wasn't the part - I just think she's a not very good/emotive actress.
As for guest star, so many but recent stand-out for me was Padma Lakshmi (?) on ENT.
How they hell she got cast I'll never know