1/2 rebar X 24 with a nut welded on about an 1" down from the top so you aren't beating on the weld. They mushroom over time from impact.
T tops for water with a range of sizes - my longest are 4'. They are great at the end of a slide wire.
Check out my post above, the weld will not crack if done like described...The weld becomes the top of the stake with the added impact support of the nut, and it won't mushroom much either, the nut stops the mushroom effect.
Leaving rebar above the nut is a guaranteed mushroom, which does aid in keeping the nut on, but looks like h3ll, and it takes more excavating to hide the stake protrusion.
Just sharing my experiences, but you may not give a crap, Lol
... as always, YMMV.