I'll be 18 in summer and hoping to go on holiday with my friends and have done quite a bit of research so far so can recommend a few places that look nice- most are beach holidays though idk if that's what you're after but:
Croatia
Fuerteventura (Canary Island)
Lanzarotte
Ayia Napa (quite party-orientated though so maybe not what you're after)
I've also been to Falaraki, Greece before with my family which is very lively but also not completely party-orientated. I'd defo go back with my friends though.
City-wise I'd say Rome, Amsterdam, Copenhagen- make sure you'll have no problem being served alcohol if that's what you're planning on though I think most places are less strict than the UK, we were welcomed into a club with free shots when we were on a school trip in Rome at 17yrs old lmao (we were separate from the teachers at the time haha)
Also recommend booking a package holiday through Jet2 rather than accommodation, travel and transfers separately because it always seems to work out much cheaper, especially if you're 17 and cba with the responsibility like me lol
But wherever you go have fun!!