I recommend one of those big six footer markers. Size matters.
There are no-frills ones...
http://www.scubatoys.com/store/detail.asp?PRODUCT_ID=FM36
And some that have bolt snaps and dump valves so you can deploy them from depth. If you get a finger spool and learn to use it, anyway.
http://www.scubatoys.com/store/detail.asp?PRODUCT_ID=LargeSafety
I got the second kind, so I can hang from it if I want to during a blue-water safety stop. You can send it up ahead of you if there is danger from boats, too. It's much more versatile, though more expensive and you have to carry a spool.
FWIW I chose EMT shears instead of a knife. I got a Storm Whistle instead of one of those tank-powered things. Maybe it isn't as loud, but it will work without air pressure and I don't want to put more fittings in an air line than I have to. I'm also cheap.
Slate--it
can be very useful. I carried one, but stopped because in practice it never
was useful. Obviously, that is a calculated risk. I also hated the way it dangled.
Anyway, as was mentioned, it depends on what you are doing.
For my easy local shore dives, I carry shears and a whistle. There is basically zero chance of being swept away out to sea so I don't take the marker, and I already explained I don't like the slate.
If I am doing a shore dive at night, or a boat dive of any kind--even a boat dive right off a beach--I add the safety sausage.
If I was out in the deep blue sea on vacation, I'd add a mirror and a bottle of water.
I would only take the slate if I knew I needed it for something ahead of time. (shrimp census?)