Own a No1, No3 (well actually a M1917, but they are damn near the same thing), No4 and a No5 its hard to choose.
I would have to say a No4. Why, because it is what my grandfather would have been trained on and used during the war.
That takes care of the emotion pick. I also like it for the sights and the sight radius, and the grooved handguards that are on some of them. It feels good to me; some texture under the fingers.
I do like the sights on the No3, longer radius and smaller peep, but it is heavier.
The holdback on the No1 is the bolt handle. For some reason I don't like the feel of it. Maybe because it is a little straighter then the No4?
The No5 is a nice light, little carbine, but it is no full sized rifle.
And like Smellie says, a long sharp piece of metal hanging off the front just feels good.