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Wider isn't always faster.

Maybe between 25mm and 28mm.
But not for every size.

(dot) bicyclerollingresistance (dot) com/specials/schwalbe-marathon-32-37-40-47

In this example the 32mm Marathon was the slowest, but the 37mm was the fastest at every pressure from 45psi up wards.
Both the 47 and the 40mm were slower than the 37mm
This was true even at higher loads.

So bigger is not always better.
Diminishing effects are real.

(I see I'm not allowed the post links until I have 10 posts, but bicyclerollingresistance is a trustworthy site)

Skipjacks

I ran Continental Race Kings when I was using a 26" mountain bike to commute and I loved them.

They are still knobbies but they are shorter knobbies that didn't create as much resistance on pavement as the standard Wal Mart knobby tread pattern.

But, I also used to cut across a grass/sand field that was usually wet and loose. That's where the slight knobby tread pattern earned it's keep. Those tires were really good (not super awesome) on pavement and really good (not super awesome) in muck. So kind of a good all around tire for most conditions. But if I was 100% pavement it would have been the wrong tire. I'd have wanted something smoother.

So your route is a huge factor in the right tire.

no motor?

I've used 26x1.5 slicks for years, first the Performance Metros and then MTBs. Both work great.

Darth Lefty 

A more thorough reading of that rolling resistance website will teach you that the tire construction is way more important than the size to the speed of the tire... but also that the fastest tires are totally unreasonable for commuting.

alias5000

Have been using 1.5s for years. Were cheaper tires with quite a bit of weight which you can feel. Next summer, I am going to try Schwalbe Duranos (1.1) which just came in.