I find Prague's network to be over-rated and slightly boring but then I do prefer modern cities.
Zürich is probably my favourite tram network in Europe and this comes with a vast network, old and new trams for variety, and a very nice city.
Brussels' most modern trams have leather seats and have to be the classiest trams I've ever travelled on. The old ones are quite quirky too. The middle aged ones are rubbish. Good expansive network and most interesting of all is that some run on Underground Metro-style routes.
Milan's famous yellowy old heritage trams are brilliant, and you can stick your head out of the back window and watch the world go by while the breeze hits your face

But the modern trams have a hideous disgusting pea green interior - walls, seats, doors, that hurts the eyes frankly, and the old ones that aren't the aforementioned heritage ones are rattly and in a bad way.
Stockholm and Gothenburg have decent systems that don't get much appreciation, but then Sweden is my favourite country to visit on the continent so I may be biased.
If you don't mind where you go, have you asked your cousin which tramway he might like to see? He might already have a favourite in mind.