I find that thin, loose, billowy dress pants are the most comfortable. Anything tight gives me saddle sores and restricts my range of motion. A downside is that you have to wrap the pant leg to avoid it getting stuck in the chain, but that's to be expected.

If you want to avoid rain paints, I can recommend RainLegs. Basically an apron that wraps around your thighs (only part that gets wet, really). looks goofy but you stay dry.