Spring anytime. I've been on holidays during the summer and also in autumn. But in late spring somethings are just opening and you don't have as many tourist around. Plus it always cheaper to visit places or travel abroad before May as the airlines hike up the prices of flights.
It does depend on where I am going. I love autumn and if I get a chance to go to the mountains or up north in October, I'm ready. If I'm going to a Carribean cruise I like to go in March or no later than April. I avoid summer travel as it is too crowded everywhere. One of the few perks of aging is being able to travel when it suites you!