As the first few posters said, what's secure really depends on where you're locking your bike up.

For my expensive road bike, which I don't let out of my sight, I carry a thin cable from REI and a tiny combination lock from Target:
Kabletek Flexweave Cable 3/16'' X 6'



I carry it because with the cost of the bike I never let it out of my sight. I use it for doing things like biking to a restaurant where I can see my bike out the window. You get twice the length out of a full length cable by looping it to itself then locking it to itself as well, with no loss in security compared to a cable lock with the lock built in.

I see REI sells this which is interesting:
REI Compact Retractable Cable Lock

But like I said...I don't let my bike out of my site. I mean I might walk into a gas station if the neighborhood is good to get water, that would be the max.

Those "folding metal" locks are more secure than a cable lock, less secure than a ulock. More convenient to carry, but they're heavy.

There's not a lot of variation in cable locks that would make one the "best". They're pretty much all the same, except in ways that are obvious by looking at them (the way the lock attaches, how thick it is, etc).