There are plenty of ISP's who's support is abysmal.
One in particular which stands out, It would be unprofessional of me to name them. Let's just say they have a country in their name. I have the patience of a saint and though as an openreach engineer I'm not supposed to talk directly to the isp I have done on a couple of occasions and that one will get me talking through gritted teeth withing 15 min's
Thinking back I can honestly say I've never heard a single bad word about Zen. In fact quite some time ago I had a bizzare fault which I ended up speaking to the ISP as the end user was on the phone to them while I was there. I was flabbergasted in how concise and helpfull they were. I'm not 100% sure but I think that may have been Zen.
Hopefully you should never be in a situation where you need any technical help, But if you do then having an ISP who uses a call centre based on a script rather than some one who knows very well what they are talking about would be a god send.