Sarah Jane Smith - and I mean classic Sarah Jane Smith. I like her in her own series and all (yes, I'm a big kid and always watch) but I like her best with the 3rd and 4th Doctor. I always liked the 2nd Romana too - and I can only have seen a hand full of stories with her in, but I liked her for some reason.
I loved Martha - sadly RTD made an epic disaster with her (mostly the whole...unrequited love thing never sat well), I also felt he just didn't do her justice in the end. I think she had the potential to become something more if they'd kept her around in Torchwood a little longer but there was no way they could do that.
I think Donna is awesome to, but I hated how everything ended with her. She was made into the most amazing person then it was all undone in an instant, and I suppose that was the whole point, but I felt cheated and...hurt on behalf of her. I foolishly had high hopes with 'The End of Time', that RTD would give her something better or take her back to herself before her mind was erased, but he just brought her back for no special reason it seemed. I kept waiting for something amazing to happen but it didn't - I think that sums up my feelings for the whole two part mess episode to be honest.