I love all of the one-day races this time of year, from Omloop het Nieuwsblad to La Doyenne. It's the best part of the season, though the weather has been a bit too nasty this year. I like when the weather plays a part, but it's too much when it forces a cancellation or course change.
I gotta vote Ronde as my favorite. Just about every classics rider wants to win it during a career, so you've always got big names ready to race. Wicked little climbs and cobbles, potentially crazy weather, and Easter Sunday. That's a great combination!