PODEMOS in Spain or Sinn Fein in Ireland. Probably nowhere else.
Not quite in Podemos' case. They don't control any regional assembly. In fact they aren't even the official opposition anywhere in the country! Even Cs has one place where they are the main opposition (Catalonia)
You could argue that Podemos is the true leaders of the opposition nationally since they did not support Rajoy while PSOE abstained, but since Sánchez won and they've moved to the left and stopped all support towards Rajoy that argument is hard to make.
For all what's worth they are in control of many of the largest mayors in the country. Of the 10 most populated cities, they hold 3: (Madrid, Barcelona and Zaragoza). However even in those 3, they weren't the largest party in either Madrid (PP had 34.6% and 21 councillors, Podemos had 31.9% and 20 councillors) or Zaragoza (PP had 26.9% and 10 councillors, Podemos had 24.6% and 9 councillors).
They only really "won" in Barcelona (Podemos got 25.2% and 11 councillors, CiU got 22.7% and 10 councillors)
Also OP said he wanted to exclude towns or something like that.
