On the fly here, from recollection...this is really hard with so many incarnations and stories...top 5:
1. Robots of Death
2. Frontier in Space
4. Ark in Space
5. Invasion of Time
Robots of Death-I liked this one, a respectable episode. I'd have given it average ratings myself, but it's a nice enjoyable story.
Frontier in Space-Not it's biggest fan, I'm not going to like. Was okay, but not anywhere near my favourite. Still a respectable episode though.
Dalek-Yes I love this one! A fantastic modern adaptation of the daleks, and shows what just one of these bad boys can do! the acting, cast, plot, everything was fantastic in that episode, a firm favourite of mine too. Might be in my top ten.
Arc in Space-Never seen this one, heard it was good though.
Invasion of Time-Been so long since I last saw this one, I can barley remember the details. I think I remember getting a little bored in it mind you. Still a respectable episode mind you.