I've hopped around many of the smaller energy companies over the years, before finally settling with Octopus. With their innovative range of tariffs and excellent customer service, I intend to stick with them (as long as their huge growth in customers does not negatively impact on their service).
As for the others, well there are two in particular I would never go back to, Scottish Power and British Gas. I only ended up with Scottish Power, as my previous supplier went bust but, in the few weeks I was with them, they messed up so badly, they ended up having to pay me compensation.
As for British Gas, well I had the misfortune to use their central heating cover and their Hive cameras, whilst a family member had the misfortune to use them for their energy supply. Both of us ended up being awarded substantial compensation for their incompetence.