Does anybody know which would be the biggest unserved city by ryanair in Europe except Eastern Europe?
IST? Depends what you mean by east - geography or politics?
I take it you don't mean BUC, IEV or MOW.
City population is misleading - e.g. ZRH proper quite small, but the hinterland is most of CH. One of the last places MOL will go. LIS - discussed recently? AMS - again, like ZRH a massive hinterland - all of 'Holland' - ie N Holland & S Holland, within easy reach.
I'm not counting anywhere were MOL has stretched the boudaries of alternatives - eg MMX for CPH, FMM for MUC, BTS for VIE etc.
Cities with 2+ airports - ie MOL can play them off? BFS is a notable ommission, I think well documented here, esp re: BHD & restrictions.