Actually, to be honest, I thought Freema was pretty good in series three. But her acting in series four so far hasn't been great - especially in the episode The Doctor's Daughter. The part when she was crawling out of the slime pit after the fish alien died - I thought that was terrible.
I know Jack wasn't a proper companion, but I loved him - not so much in Torchwood, though.
On another note - since watching Firefly, the word companion has taken on a new meaning!