I moved to Octopus. Yes, OVO return any credit in a timely manner.

If you want to move to Octopus I have a referral link that will give you £50 in your account.

https://share.octopus.energy/grey-crick-824

You can call collections and ask for a freeze of your DD at a lower level or a return of some of your credit just before your switch.

0800 0699 831

The rules for refunds after switching are the same for all suppliers as you probably already know. 

Up to 6 weeks for a final bill. Then up to the 10 days for any credit refund following the final bill.

https://www.ovoenergy.com/help/leaving-ovo

These are industry standard times that all suppliers have to meet. 

Miss either of them and £30 compensation is normally automatically due.

Obviously depends on what you want from a supplier in terms of tariff etc , the Which report is worth a look at, although not perfect 

https://www.which.co.uk/reviews/energy-companies/article/best-and-worst-energy-companies/which-energy-survey-results-ajqM43e6ycY8

Trustpilot is another useful proxy, the big suppliers left for example are

OVO 

https://uk.trustpilot.com/review/www.ovoenergy.com

EDF

https://uk.trustpilot.com/review/www.edfenergy.com

Octopus 

https://uk.trustpilot.com/review/octopus.energy

Eon Next

https://uk.trustpilot.com/review/eonnext.com

British Gas

https://uk.trustpilot.com/review/www.britishgas.co.uk

Scottish Power 

https://uk.trustpilot.com/review/www.scottishpower.co.uk

Shell 

https://uk.trustpilot.com/review/shellenergy.co.uk

Utility Warehouse 

https://uk.trustpilot.com/review/uw.co.uk