A really big, heavy weight would work. But it would be hard to hoist and hard to store on a boat. So anchor designs have evolved to minimize weight, using shape as a substitute for it. An anchor not only has to grip the bottom, it has to maintain that grip in a storm, when there will be hours of pulls and jerks on the anchor rode. And it has to maintain that grip when the wind or the tide shifts, causing the pull to change direction, often 180°.
Over the years, many kinds of anchors have been invented. In the modern boating world, the variety available is wide - each type has its advocates, some of whom are almost evangelical in their zeal. I do not want to stir up controversy, so in what follows, I am not advocating; I am describing: what has and has not worked for us, and for our neighbors. Also, different kinds of anchors work better with different types of bottoms. Our cruising area is mostly mud bottom, with the occasional sand, so these experiences reflect that.
Speaking of mud, with a mud bottom, there is a minimum anchor weight. If the anchor is not heavy enough (regardless of its shape, or the size of the boat from which it hangs), it simply will not bite effectively into the mud, no matter how clever the design. Instead, it will skip along the bottom ineffectually, or just the tips of the flukes will catch, leaving you to think that you are hooked. But the first time the wind comes up, it will pop out. I have a great "I learned about sailing from that" story about one such incident (with a Danforth) that happened to us. I suspect that that minimum weight is about 20 lb.
Around G Dock at Shilshole, these are the types of anchor most commonly seen:
the anchor will "fly", skipping along the bottom.
We have two of these on Eolian, one as a bower, and one stowed in the lazarette for use as a stern anchor.
This one on Ghost surely weighs way more than my 20 lb recommendation, above. There is now a lightweight version of the lightweight Danforth, made of aluminum by Fortress. Tho they are easy to handle on deck, beware!
We have never used one, but I think their popularity among experienced boaters speaks well for them.
The Navy TypeIn the sizes used by pleasure craft, I cannot recommend this anchor. (As an anchor for an aircraft carrier, at 60,000 lb, it is apparently a good fit.) We have had two boats on G Dock with this type of anchor drag and sustain damage. Both subsequently changed out their anchors for one of the above types.
The Delta Type
Whatever type of anchor you have, be sure to pull on it, hard, with the motor before you shut things down and break into the liquor locker or jump into the dinghy. If you want to sleep well, you don't want to wonder if the anchor will break out should the wind pipe up.