My favourite theme to date has to be Aerials V2 by Xavier, with a selection of blue, green and violet to choose from. I personally have the blue variant on my E71 and a few particular features I love are the way the loading bar, when transferring data or refreshing feeds, swings from side to side in such an exquisite manner.
Also when entering a number on your home screen, the back panel becomes transparent. You’ll need to install the theme to understand exactly what I mean. Aerials V2 also features the Full Touch Icon Pack with over 400 3rd party application icons. There are six version packs included in the download for every S60v3 FP1 and FP2 device and you can download the theme here. Remember, Xavier also provides the themes to use with your default S60 icons.