In my experience, through extensive and exhaustive research, the EE rankings in order of performance, in general terms are:
Polish - great attitude, top performance
Ukrainian - great looking and if you find one who is not purely after the cash, you hit the jackpot
Russian - best looking, often "princesses", inconsistent delivery
Czech Republic & Slovakia - dirty bitches, in a good way
Baltic States - the younger ones often top notch
Hungarian, Moldovan & Romanian - occasional good 'un