Paris would probably be the warmest of the lot you mentioned, but still not warm.

Try weatherunderground.com to get an idea of historical weather patterns for a particular time period.

My preference in terms of climate and perhaps my favorite city would probably be Rome, but that's expensive for hotels, depending on what you want. For example, I can't imagine $450 in London being necessary when Priceline offers such great bargains on 4* hotels, so you may have very high hotel standards (which is very expensive in Rome).