I'd go for Lisbon, Madrid, Porto then Venice in order of preference, but unless money is no object, a good start would be where you could get reasonably priced flights from a convenient airport at a convenient time.
For example, if flights were £200 each to Lisbon, but £70 each to Madrid, that would be a strong pointer towards Madrid.
Likewise if all the flights to Madrid set off or arrived at stupid o clock whether early or late, that would put me off.
Or if there were no flights to a particular place from my local airport, I probably wouldn't bother going.
Then you have to apply a similar thought process to accomodation. Can I find something in a good location for a good price with good reviews.
Applying the above often cuts down the options considerably when there are several places you could equally happily go to.
If you look at a different time, you often get different answers to the flight and accomodation price conumdrum, so it's not like you're striking anywhere off the list forever, just this time.