Best is very subjective as they all work pretty well.
I personally like the autotopoff.com double switch matched with an aqualifter as it is relatively cheap, reliable and easy to install.
Best DIY setup is floatswitches.net. Cheap but you have to roll your own. I think twopartsolution.com sells one of their all in one setups with two float switches and other parts.
Expensive falls to a dosing rig like the LiterMeter but these pumps are ultra reliable and last a long time. Way over kill IMO unless you plan on using the LiterMeter for other dosing and the topoff setup is still pricey.
Cheapest is an IV drip on a container. Very cheap, very reliable. Only issue is you have to adjust the drip from time to time but otherwise works really well and very little chance of dumping so much top off it causes tank problems.
Nice cool looking setup is the JBJ unit. Little more than cheap options, but works well.
Most overrated... Tunze's system the Osmolator. Works really well but expensive and overkill as well.