Got to go with ITLD on this one: pussy willow is fantastic for bees, and in my experience a bit more reliable than crocus, which tend to "fall over" pretty quickly, what with sparrows pecking them and big, queen bumblebees rummaging around in them... Likewise Mahonia is a good reliable early plant for my garden.
If it's definitely bulbs that are needed then I'd recommend a mixture, as each year seems to favour a different sort. I reckon snow crocus are the best (Crocus chrysanthus) but mine faded out this year, whereas the big purple stripey Dutch ones were OK. Chionodoxa are great but seem to always get eaten by something in my garden. For shadier areas snowdrops all seem to be good, and winter aconites and wood anemone (mine are all blue Blanda type) are also really good for pollen.
For me, though, the best early flower isn't a bulb: my bees went absolutely crazy over pulmonaria last winter, and it flowered - and was visited - without a break from the new year up until march. Even on days with frost they still pottered out to it occasionally. It's a great thing to put in a shady, sheltered area quite near to the hive.
