Of the tyres I have, my GP4000 IIs are 25mm and the Gatorskins and Marathons are 32mm. I also have a pair of 28mm Ultra Sports, which I think are decent value as cheap tyres but not great. So the size makes a big difference.
For country roads I really like both the GP4000s and the Gatorskins. The fatter Gatorskins are comfier over bumpy ground and have better puncture protection*, and I use them on hard-earth tracks too. But the GP4000s really feel great on good roads.
The Marathons are mainly for damper times of year when my off-road tracks are softer and I need the tread. On good road surface, they definitely feel more sluggish. On the good side, they seem like they'll last forever. I rode about 600 miles on mine before swapping to the Gatorskins for the summer, and that hasn't even fully worn off the thin molding line on the rubber yet - barring catastrophe, they should last a good few thousand miles. For any loaded touring (which I used to do in the past and hope to do again) they'd definitely be my choice.
Michelin, Vittoria? They have their fans too, but I can offer no opinion as I haven't tried them. If I had enough spare cash that I could buy tyres just to try them, I would. But my overall thought on modern tyres (especially having come back to cycling only a couple of years ago) is that they're much better than the tyres I rode 20 years and more ago, and I expect the best from the top manufacturers are all excellent.
*Talking of punctures, it's perhaps ironic that I had a tyre wrecked quite recently by a glass puncture - and it was a Gatorskin, which only had a few hundred miles on it. But it was a serious chunk of glass which went right through everything and sliced through enough of the casing threads to make the tyre unusable, and I doubt any pneumatic tyre would have resisted it. So on that bike I currently have my remaining Gatorskin on the back and an Ultra Sport on the front as a compromise until I get round to ordering a new Gatorskin.
