The Halkett Place shop was visited by judges in June, and staff learned last week that they had topped 1,985 other entries for the prestigious title.
Their national victory came after they won the southern region award to go into the final.
They were also runner-up in the most improved retailer section.
They have been presented with two plaques to mark their success.
General manager George McLoughlin said: ‘We rebranded ourselves as Euronics 18 months ago and we’ve grown massively since then – our turnover was close to £5 million last year and we’ve got some big clients under our belts.
‘We opened a new cash-and-carry store in Minden Place 14 months ago and a second warehouse in St Martin five months ago.
It means anything we have in the shop we have in stock and we’ve got dedicated teams for every part of our business.