I'm a huge fan of Kate Mulgrew. I can't speak to her talents relative to Oscar-winners, but I can say without a doubt that she's on the higher end of the spectrum of Trek actors. For example, I think she's a much better actor than Avery Brooks. She really seems to be living in whatever scene she's shooting, not just hitting her mark and reading her lines. It's the little things she does--body language, facial expressions, moving around--that just make her pop more than others.
And maybe the scene with Tuvok isn't a fair comparison, because the finished Mulgrew version has multiple cameras, while the Bujold one only has one camera, but I like how Mulgrew moves around, making Tim Russ also move as he's talking to her. You get a much better feel for the friendship between the two because of little things like that.
Having just watched DS9, I can honestly say that Sisko as a character didn't come alive for me the same way that Janeway did, mostly because of the acting. Yeah, Sisko punched Q, and he did lots of other badass stuff, but most of the time he just stood there reading his lines projecting absolutely no emotion whatsoever. That scene where his girlfriend threatens to break up with him and he just kept on eating his dinner, expressionless, convinced me he was completely lacking in empathy.
Janeway, on the other hand, seems profoundly empathetic, and it's not just because of the dialog, but because of the acting. It's very attuned to the kind of stories they were telling and to the medium. From the start, Mulgrew absolutely nailed what Captain Janeway was about, which I think puts her ahead of everyone but Shatner in that regard. Sir Patrick, of course, is beyond category, but I've got to admit that it took him some time to come to grips with Picard. Mulgrew doesn't seem to have that kind of learning curve--she's not awkward or hesitant in the least here.