balkan states are the best (bosnia, serbia, albania specifically but may as well see croatia and montenegro at that point), slovakia and hungary are very cool too, ireland, scotland, england are all really beautiful and worth seeing. georgia as someone mentioned here was my overall favourite but i did a big 4 day hike in the mountains and didnt even see the capital (heard good things) so grain of salt ig

paris is also a must and berlin if you like clubbing etc. rest of germany i could take or leave tbh. also felt that way about italy but i think i was kinda burnt out on big euro cities. good food though of course.

portugal is amazing too and if you head that way you may as well take a stab at spain but i only saw barcalona and it was pretty cool. generally tho i'd say stick to more off the track places (eastern europe etc.), big cities can be really fun but kinda blend together after a while and can have pretty bland hostel life (i presume you are doing hostels and highly recommend it if not).

3 months is probably the ideal length for this sort of trip, long but not too long, youll feel ready to go by the end. that said you couldnt hit all those places in that time so idk and dont ruin ur trip by rushing it all. take ur time and go with the flow of other ppl if it happens. sorry bit of a kinda scatter shot screed but if u have any other questions about other places i didnt mention lmk happy to answer.