By European do you mean EU member states or everywhere in Europe?
Not that it matters I suppose.
My choices, Tallin, Athens, Heraklion, Stockholm, Dublin, Helsinki and maybe Amsterdam.
Well for me it would be more about what the countries have to offer more than the city itself. I guess it depends on what your reasoning is but for 18 months you would get to experience summer/winter and would have the whole country at your disposal.
Amsterdam might be the one exception to that though, it's an absolutely lovely city and I could think of worse places to live in if you had too.
Like I said it's more to do with the rest of the country than the city itself although the city from the short amount of time I spent there is very nice too. If I had a choice I wouldn't want to live in the bounds of a city center but that's just me.
They are English speaking too so that is a bonus but you do have to get your ear in.
Again it depends on what is important to you and why you would be moving in the first place.