EE
I have to admit that the first thing I do with any phone now is root it and put a custom rom on it. Not only do I get rid of the crap put on there by the operator but I have far more control over the software on my phone, what I can do with it and far more frequent updates. (Must admit, XDA forums are a godsend).
As for EE they are ok as long as they leave me alone. I generally get the phone from Carphone Warehouse or similar and I avoid 4G. I still think it is a waste of time and I would rather have the battery life from 3G as the data rate is more than sufficient for what I want. On top of that it meant I could get unlimited minutes and texts with 3Gb of data for £40 a month, with the latest smartphone...