I've covered Jaku before but that was over a year ago and since then a ton more icons have been added. Many icons have also been updated to look even better than before.
There are so many themes on Cydia that'll it'll take way too long to go through them all but out of all the ones I've tried Jaku is easily the best. I think it is the best and most stunning theme for iPhone. The great thing about this theme is that it is constantly being updated with more and more icons. More third party apps are being added to the list constantly. There is also a Retina iPad version on the way. So iPad users can also enjoy this gorgeous theme.
The theme doesn't just change icons though, some of the elements of the OS are also changed. It brings down the ugly glossy look of iOS and makes it a bit flatter.
The theme is only $2.99 which is a very tiny price to pay for such an awesome theme. Simply search Jaku in Cydia and you'll find it.
Also check out Circles, it doesn't have as many icons and doesn't change the system UI elements, but it's still a very good theme.
You can find the wallpaper used here.