Having visited every single one of them I would say:
Slovenia: it was an undiscovered gem when I visited the place about 20 years ago.
Romania: as above
Italy: oozes history, character and atmosphere.
Spain: I avoided the place for many years after images in my head of the tourist resorts. Great country, especially Andalucia.
Iceland: magical
Holland: the people. It's like home.
Germany: Lived there and became hooked on the people (straight as a dye), culture and language.
San Marino: very chocolate box.
Austria/Switzerland: stunning scenery
France: a delight on the eye.

Weirdest European countries visited:
The eastern strip of Moldova which is still under the control of Russia. Very eerie.
Northern Macedonia: very poor bit with a regime that pumps money into very vulgar and ostentatious statues in the centre of the capital city.
Azerbaijan: oil money seems to have trickled down to a very small minority and the rest of the population seemed very poor.
Albania: I went there when it first opened up and there was nothing to buy. There were hardly any vehicles on the road and some of them I saw had replacement side windows made from plywood! It was not uncommon to see ground floor flats surrounded by pigs.

Surprises:
Odessa in The Ukraine: almost Parisian in nature.
Minsk, Belarus: incredibly sophisticated despite being ruled by a despot
Bosnia: a beautiful, non-functional* country. (*Still beset with profound factionalism)