Personally I like:
simplicity like Heidfeld's,
nice contrast like Kubica's,
strange two side idea like Liuzzi's,
Red Bull can idea like Vettel's,
metalic version like Glock's,
clear design like Di Grassi's,
Unfortunately in this gallery there are no back side and top pictures of the helmets.
Here You can find this: Helmets at Bahrain GP
In some cases interesting.