Partners in Crime with Smith and Jones close behind.
Before Easter 2008, I wasn't a fan of Catherine Tate, I've seen some of her sketches and I loathed Donna in The Run Away Bride, but at the end of Partners in Crime, I thought she was brilliant, she had grown up and was a breath of fresh air after Rose falling in love with the Doctor and Martha falling in love with the Doctor, she was a mate to him, and it bloody well worked for me. Also, it was fun, it had a spaceship straight out of Close Encounters of the Third Kind and overall it was fantastic.
As Steve Mottaf has said, Smith and Jones was very subtle in it's technical greatness and I understand it, I just feel that Martha wasn't as great as she was made out to be (especially by Rusty himself) although the Judoon essentially being Vogons was pretty cool, but on the whole, it was just ok. Oh and I can not stand that Radio One DJ in it, but that might just be me!
As for Rose and New Earth, it's been a while since I've watched them both, but I prefer Rose over New Earth, New Earth is just a pile of unadulterated shite for me.