what is the most secure bicycle combination lock?
I think ABUS is one of the most respectable, perhaps THE most respectable bike-lock manufacturer around. They have even some numbered levels of protection, the higher the level the higher the protection, even agains powertools (because of special steel alloys, and such).

They have some combination padlocks, which seem very interesting. Take a look at these 4-digit (ten thousand possible combinations):


Reinforced padlock
Chain-lock with combination and key
Cable-style lock
Linkage style (this one is awesome)


There are other options in their site, but I think these are the most robust combination ones.

EDIT: just to complement, I think combination locks are a good option for errands, but not for whole-day or part-day parking out of sight. I suppose the OP intends to do so, since he mentions "in the street" and "keyless".

For example, I use to go to the supermarket by bike, and I think a combination would probably demand time enough to open so that someone wouldn't risk an opportunistic theft.

Now for parking at, for example, a train station full of annonimous bikes (none of these parkings in my city, yet), I'd go with the keyed ones, no doubt. A combination one would be an invitation for the thief to earn a bike AND a keyless lock "combo".

As a final thought: does anyone go out without his own home keys? Would a lock key be too much to put together in the home keyring? Well, just a thought!

Hope this helps!