GP4's are lovely for 28mm and smaller. But I'm a big fan of Marathon Supreme for 32mm and larger (I run 35mm). They are incredibly supple, not quite as fast as GP4's, but much quicker than touring tyres. Fantastic grip on any combination of wet, dry, cold, warm. I've had no punctures on crap chiltern lanes and a fair chunk of byway and canal paths. I'm not particularly quick at Audax (under 11 hours if I'm lucky) - but compliant tyres mean I can still function as a human the next day with no aches or pains.