Showers pass has an excellent reputation. I wouldn't call it "best value" though, arguably "best" but at a stiff price. I think OP might be looking for something along the lines of 80% of the quality of showers pass (or more), at 50% of the cost (or less).

I bought a rain poncho (Knalla) from Ikea for like 6 bucks. It packs into its pocket, so that's convenient. Unfortunately, here in San Diego we don't get much rain (freak storms last weekend, but I didn't have to ride in it!), so I haven't tested it out yet!