Which mobile brand is doing the best job of engaging customers online?

Carphone Warehouse has actively set out to compete online with the leading mobile telcos to capture customers (and their data) and act as an aggregator in the market.
In response O2, Three, EE, and Vodafone along with many of the challenger brands have responded by keeping their most popular SIM-only products in their own channels and invested heavily in trying to attract and engage customers directly into their sales funnels. 
So who is engaging the customer and who is left hanging on?Traf