I've got a lot of great betas (seven or eight I routinely share with), but my first one (who still reads for me) is the best.
She was incredibly generous--after reading the crappiest query on Evil Editor's blog, offered to beta the MS (plus re-write my query). This was when I was just starting out, and the MS in question was a huge rip-off of another series.
She gave me so many helpful tips--up to five pages of suggestions for a four-page chapter (sent back the next day!). She read the whole MS. She re-wrote parts for me, made amazing suggestions--tough ones that made me cringe, but which she delievered with grace.
She even revised my originally crappy query (no nibbles from agents) to one that got me 10 or so partials and two fulls (out of 27 queries).
Well, it didn't work in the end because my novel was awful, but you get the idea. She helped me with my voice, helped me improve SO much, it's not even funny. Now my writing's way better, and I honestly have to thank her for that.