I don't really drink "seasonally", but if there is one beer that I associate with spring it's Bock. I love the stuff...including the American ones like Yuengling, Genesee, Shiner, and others like it that seem to be making a resurgence.
Part of it is probably a sentimental association, too: The very first sixpack of beer I ever purchased for myself, back in the spring of 1969, was a Bock beer. I looked forward to it every spring after that.