I've pretty much abandoned what many of us once considered to be the de facto 700 x 23 road bike tire. So several years ago I started going with the widest tires that I could comfortably fit on everything except for my very Italian Freschi. (That one has zero clearance for anything wider than a x23, and it would look pretty weird even if it would fit wider.) Hopefully I don't sound like I've been drinking the Kool-Aid, but the introduction of supple Compass tires - especially the EL versions - have simply been a revelation for me.

My data points include the following:

'66 Paramount - x28
'89 Paramount - x25 (no room to go wider)
'80's Freschi - x23 (same story - tightest seat tube clearance you'll ever see anywhere!)
'71 International - x38 - Compass tires sure made a difference over the bulletproof things I had on there for a while
'70's Carre - x32 - Compass tires
Boulder - x32 - also Compass tires
L'Avecaise - 650b x 42 (not sure this counts because it's purpose built for wide tires)
'46 Hobbs - x28

I really like x32 for the combination of sportiness and comfort.