you have two major factors to consider: 1) weather and 2) tourists.
1) the summer is extremely hot pretty much anywhere, at least for my taste. i agree with DS that may is nice overall. don’t go anywhere north in the winter months, including october. i was in venice this march and it was in the 40s and low 50s some days. i think anytime in may is the best option, maybe even late april.
2) summer months are bad for tourists in any of the major cities (rome, florence, venice). being in venice in the summer is like being at disneyworld (i.e. overheated, overpriced, and overcrowded with americans). again you can avoid most of this in april or may probably. or you can visit some of the lesser-known cities. for example, padua (padova, in italian), is a gem of a city about 45 minute train ride outside venice. unlike DS, i rely on train travel in italy which i find to be overall reliable…having been a passenger in a car in italy, i can’t imagine having the guts to drive myself around; plus, i’ve heard (but can’t corroborate) that thieves can spot rental cars to single you out as tourists.