In reply to SiScrambler:
If it's wet everywhere and you have the usual south-westerly winds, Cwm Idwal is usually the most sheltered place to scramble. The direct start to seniors ridge or Idwal Buttress are both fine in the wet. The east face of Tryfan can be sheltered from westerlies, but it is a high mountain and can be rough on the top/descent depending on the wind.
Tryfan Bach is reasonable in the damp, but stick to the crack lines rather than the slabs, these aren't fun in the wet.
Tremadoc can be dry when the mountains are wet, but if it's at all damp I find it unpleasant to climb on. Holyhead mountain catches the sun and dries quickly, but with westerlies again it will be unpleasantly windy and cold at this time of year.