Others can talk about their favorite cities, but you have far too few days for Paris, and not enough for Amsterdam or Venice.

Part of the fun (call it what you like) of a trip is to get out of a city and see what the local folks are like.

Your trip sounds like one of my mother's tour groups she goes on "we saw everything, sort of".

In the period of time you've got, pretty soon every city will begin to seem like a blur.

Seeing less is often seeing more. Cut down your itinerary by half.

All this overnight travel... have you actually checked on train schedules? It doesn't take overnight to travel between many of your planned cities.... Amsterdam-Brussels is a couple of hours. Ditto Brussels-Paris.

But by the time you check out of your accommodation, get to the station, buy a ticket (lines can be long, take the trip, find your accommodation in the next city you've expended a lot of energy and time. ;^(

I'd strip out Milan and Nice as well as Brussels that frees up 5 days, I'd think about Porto as well as it doesn't really dit well with the other cities. I'd try and get an open jaw flight to go back to Toronto from Paris.

Now please tell me you are not doing this in July and August?

Less is more.

Most people on this forum prefer a slower approach, so that is the feedback you are going to get. You really just need to think about what you want out of this trip--if you want a grand overview of western European cities, then I don't see any glaring problems.
I have to say this would wear me out, but I am 39, and that is just my personality. When I did my backpacking trip, it was woefully short and I loved the overnight trains, but you can expect to need some r&r at some point. If I was going to critique, I'd say try to build some of that in.
My other critique would be that it is all cities, but again if you are looking for that grand overview, then it is also probably easier to get from place to place without adding in rural areas or small towns.
have a great trip!

Actually, planning a trip in terms of days, rather than nights, isn't the best way to get a real sense of what it will be like, because after you figure in the packing up time, getting to the station time, train time, getting oriented time, finding your accommodations time, unpacking time, getting a bite to eat time, etc., well....there goes the day.

Just to clarify, I found a very good deal on a flight, so I have already booked my flight from Toronto to Lisbon on April 30th, and from Lisbon to Toronto on June 13th.

My plan with this trip is to do a "must see" sight seeing tour of Europe. I want to see all of the major places but still have enough time to walk around town and see a few local places as well (specially in Italy).

As for the night trips, I looked into trains but it was out of my budget. Instead I plan on taking buses. I found night bus trips between most cities. On the longer trips I plan on just sleeping on the bus (like from Venice to Budapest) while for the shorter trips I'll take early morning buses instead.