To be honest, there really isn’t a BAD time to visit the Columbia Gorge. There are a few months in the winter, when ice can be an issue, but even then the waterfalls are beautiful when they are half frozen over.
For wildflowers, the best time to visit is generally in late April through June. April is also peak bloom time for the orchards of the Hood River Valley and around The Dalles.
For the best weather (i.e. least chance of rain) late July through August are always a safe bet.
My favorite time of year in the Gorge is the fall, from about mid-September through the month of October. Most of the summer crowds are gone, the fall foliage puts on a stunning show (consistently one of the best in the US), and the bounty of the harvest is at its peak in the Hood River Valley. The weather is usually still pretty pleasant too.