I have used GP 4 Seasons, about 3 sets including doing PBP on them
They are grippy and fast. The 28mm flavour (actually 26.5mm) has plenty of air in it for comfort. But I don't use them now. Why? After 2500km they have enough nicks in them that they are not puncture resistant at all. I expect a puncture every 200km after this. Given that they aren't cheap tyres this isn't durable enough. Otherwise good though
Someone else mentioned Michelin Pro Race 3. Now, I've used Pro2 25mm extensively and had a good experience. They are incredibly fast and grippy. Comfy too. I've never had a puncture on them, despite doing a wet Bryan Chapman on them down some very grotty lanes.
The Pro3 is in many ways a better tyre than the Pro2. It is as fast and definitely grippier. I had some 23mm ones on my commuter bike. Difficult to compare them for suppleness vs the 25mm Pro2. They were very comfortable however. But they only lasted a week before being destroyed. Now, with a sample of one set of tyres over one week this isn't the best way to form a view. But the word on the streets is that the Pro3 is not as durable as the Pro2.
At the moment my long distance bike has 25mm Michelin Krylions on it. These aren't as supple/comfortable or grippy as Pro Race but feel much stronger. My tyre thoughts at the moment for 600km+ are
1) my NoS Pro Race 2 25mm- I have 4 with minimal wear and one still in the pack. Unlikely to find any in the shops though
2) Specialized Roubaix Pro - I got a pair of them with the Roubaix, they seem quite nice and are 25mm. Seem expensive to buy
3) Schwalbe Ultremo 28mm - Haven't seen them IRL but this is a 28mm tyre on a proper race/training compound. Take the mudguards off and they should (just) fit on the Roubaix. Difficult to source in this country but starbike.de do them
4) Michelin Pro Optimium - As discussed above, the Pro3 doesn't last as long as the Pro2 and the Krylion isn't as good a tyre. But Michelin have started making a long distance "Pro" performance tyre, only in 25mm. Still haven't tried them. Slightly costly but widely available
5) Panaracer Pasella - Not as fast as the above but a nice tyre. There is a folding bead version that is quite light. Used these in 28mm flavour on LEL 2005