Onions, basil, oregano
I remember this from last year - the oregano takes forever to germinate, and the seedlings are so impossibly tiny that I walk around for weeks convinced that they've damped off and I just don't know it yet because I can hardly see them. Then, just when I'm on the verge of tossing them, I notice that they've finally got their first set of true leaves and are on their way to taking over my herb bed.