I really rate the Weatherbeeta freestyle combo Light, got a brand new one on ebay for £40.00, my mare is quite chunky but i find them a good fit, waterproof, and dont rub her mane!
A lot of people really like Premier equine combos too but the lightest in that i think (could be wrong) would be 50g/70g, worth a look though!
I've got a Weatherbeeta Landa Lite which has attached neck, very good, also have a Shires one, equally as good. Try www.equestrianclearance.com they have some good bargains. Also recommend PE rug, fit my two TB's really well.
My TB has a Mark Todd and its so brilliant he now has several MT rugs- they are 100% waterproof seem pretty tough fit brilliantly and dry off again quickly.
They are slightly more expensive than some but i would say well worth the extra, i got mine off ebay a good bit cheaper than in the shops.