It is amazing in the rain, water beads and dries quickly. The fabric is called "Zorander" and is a %100 merino wool by Loro Piana. It isn't too heavy and has a removable liner, so you can use it in a fair range of temperatures.
The bonded cotton that Mackintosh uses is completely waterproof as far as I can tell. It's a bit stiff and it wears warm but it's perfect for cooler weather.
Best for what? repelling rain? making the wear look good?
My Aquascutum is 100% cotton and wears well, but I'm not sure about it's ability in a downpour. It looks the best, even when I compare it to a Burberry that's a cotton/poly mix.
My Mackintosh is completely waterproof, but feels stiff
While the Loro Piana Storm System has a very nice feel, I'm not sure about long term durability of it's waterproofness.
You will want something synthetic, whether its in the thread, like a poly-blend, or in the treatment, like the Storm System stuff, if you truly want to keep the rain out. Otherwise you'd have to wear the original rubber Mackintosh.
If it's just something to wear in Spring, you can get away with a light, papery cotton -- though here again the best ones have a bit of synthetic in them to make them that much lighter.
I bought a great 100% cotton really slim fitting mac from All Saints a year ago. The quality is of course hilariously questionable but the fit persuaded me to go for it. I put some waterproofer (Can't remember the brand now but you can use it to reproof gore-tex or E-vent jackets) and now it's pretty reliably shower resistant.
