Ceredigion, one of the new unitary local authorities carved out of the old counties in the principality this year, boasts only seven secondary schools, the largest with around 1,100 pupils.
This week's figures show that 51 per cent of the largely rural authority's 15-year-olds passed five or more top A* to C grades, three points ahead of the nearest rival Powys.
Ceredigion, centred on Aberystwyth and with many Welsh-speaking schools, also came near the top for pupils gaining at least one GCSE pass, with 93 per cent.
Director of education Roger Williams puts the success down to education being held in high regard and to the fact that many schools are relatively small.
"There is a Welsh tradition of education being important, and perhaps it is stronger in rural areas," he said. "We have fewer schools than some areas and many have small classes. It means schools and the authority can work closely together and that encourages an atmosphere of improving standards."
The latest overall results in Wales have again improved this year to a new all-time high, with 42 per cent of 15-year-olds gaining five or more top GCSE grades, one per cent up on last year.
A- and AS-level results also stand at record levels, with an average 16 points scored per pupil.
This year figures have also been published showing how each school has performed against the target set under the Bright Future policy established last year, which is for half of 15-year-olds to gain at least a GCSE grade C in mathematics, science and English or Welsh by the year 2000. The average for Wales is 32 per cent.
Wales Secretary William Hague said: "The inclusion, for the first time, of each school's performance against the Bright Future target shows we are making steady progress in equipping our young people with basic literacy and numeracy skills."
But he warned that "much hard work lies ahead" if the Bright Future target was to be reached.
Bottom of the table for pupils gaining five or more top GCSE grades was Blaenau Gwent with 28 per cent, a point below Merthyr Tydfil with 29 per cent.
Ceredigion also had the highest number of pupils meeting the Bright Future threshold of top passes, with 39 per cent.
Bottom of the Bright Future league in Wales were, again, Blaenau Gwent, with 21 per cent and Merthyr Tydfil with 24 per cent.
Cardiff, Ceredigion, Pembrokeshire, Powys and the Vale of Glamorgan achieved the best scores for those doing two or more A or AS levels, all with 17 points on average.
Cardiff had the highest percentage of candidates gaining the top three grades in two or more A or AS levels, with 58 per cent, followed by Swansea with 56 per cent.
The Welsh results were slightly behind those in England, where 42.5 per cent of pupils gained, on average, five or more top GCSE grades. The gap is thought to be due to social and economic differences; Wales has more industrial and rural working-class populations.