Cinque Terre is one of those places people go because they see a picture, probably an image like the Manarola sunset image below. It's a lovely sight. Cinque Terre has a lot of tourists. And when I say a lot, I mean it is hard to walk down the main streets during the day because during the day the hoards of the tourists have descended on Cinque Terre like a plague of locusts. The best time of day in Cinque Terre is in the evening after the hoards go wherever the hoards go for the night.
Calling it the "Five Towns" is a bit of marketing genius because you feel like you should visit all five. And if you've got the time, sure visit all five. It took us a day and a half to do so, going between the towns by train. However, if you are short of time, here's my ranking:
I came away with mixed feelings about Cinque Terre. If you are like me and loathe being in a swarm of tourists, the off-season might be the best bet. We visited at the start of the season and it was crowded. In the summer months, stay out late into the evening stay late into the evening so you can enjoy the atmosphere. One thing to note, if you stay in one of the villages, the roads are very steep and you'll likely be lugging your luggage up a steep hill. We stayed in La Spezia which is near the end of the train line which worked out well for us.