I'd say Rome and Florence, but my biases in this sub are apparent Maybe a little more objectively -

Florence if you're more interested in art, shopping or Tuscan food/wine
Venice if you're more interested in architecture, a very unique history and a city situated like pretty much nowhere else (no cars, no roads, just canals)

June is going to be high season for tourists no matter where you go. Florence isn't too bad even with high traffic. Venice is bad during the day, but you can find yourself fairly alone after the cruise ships leave each night. No clear winner in that category.

Both cities should be interesting for 18/19 year olds too.