The 90 per cent result equals the firm’s record score last spring in the twice-yearly poll and was 4 percentage points better than last autumn.
However, it was beaten by cross-Border operator East Coast, whose rating improved by four points to 91 per cent on a year ago.
Virgin Trains’ score fell by 2 points to 90 per cent.
Aberdeen-based FirstGroup, which is battling to retain the ScotRail franchise from next year, said improved performance and better timetables had been “key drivers” in achieving its score.
Managing director Steve Montgomery said: “We have improved in the vast majority of areas, some significantly, so our focus now is on maintaining these, while re-doubling our efforts where our scores are lower.”
Passenger Focus said there had been significant improvements since last year in satisfaction with connections with other trains - up 10 percentage points to 85 per cent - and with the helpfulness of staff, up 9 points to 85 per cent.
ScotRail’s scores were up in 24 areas and down in six.
Nearly 1,100 passengers took part in the ScotRail research between February and April.
Satisfaction among CrossCountry passengers fell by two points to 82 per cent, and among First TransPennine Express passengers by one point to 85 per cent.