I agree it is so difficult to think of one. How about one from each season.
Winter has to be the snowdrops - so cheering to see their happy little heads nodding to say spring won't be long.
Spring - has to be the daffodils - bright and cheerful
Summer - Roses - so beautiful to look at during the day and in the evening you can still smell their lovely scent.
Autumn - Rudbeckia (although I know more late summer) but they do keep flowering til the first frosts.