While I agree with the general consensus that anything waterproof will leave you as a hot sticky mess, being sensible about the rain is recommended.
Unless it's really chucking it down, I'd recommend avoiding goretex and other water proofs. Windproof is where I think you'll get the most benefit, in most showers they cope ok and if they do get soaked, well you're wet but the wind isn't an issue and it's the wind that is often the big problem of being wet. You'll generate loads of heat moving around. There is a bunch of wise people that recommend windproof under your camo of choice, it's much harder to get it wet, still windproof and if your out layer gets soaked - again, not the end of the world.
I find baseball caps are super useful for keeping the worst of the rain off your glass/goggles and if you're comfortable with mesh, having a pair of mesh goggles in your bag for those super wet days. Some people like boonies, I have never got on with them but I have a weird head.
I find trousers, socks and footwear more important for general comfort. I don't always recommend waterproof boots, they can be good but not cheap, need maintaining and often heavy. I like having a pair of sealskinz in the bag, when it's wet enough to soak my boots I just wear those, not great for 2-3 days straight, but an average skrimish day is fine. If doing multiple day events, good waterproof boots are sensible, talc and sock changes.
Gaiters are massively under appreciated, often it's not even that rainy but your feet and lower legs get soaked from wet grass - gaiters are your friend. When it comes to trousers, a choice that dries relatively quick (not jeans) and isn't cotton (poly cotton is ok I find), so your average camo pants work fine, some clones of crye pants aren't great and it's this sort of thing where having real ones can make a big different to your comfort.
A spare pair of gloves is a nice luxury as well, putting wet gloves back on after lunch just sucks, as does getting shot in a freezing cold, wet hand that doesn't have a glove on because your only paid is wet. Check out mechanix, i think their originals line is awesome and well worth the cash. When wet I find they still keep my hand less cold than without them.