Oedogonium. It spreads rapidly and covers leaves with a dense coating of 1/4 to 1/2 inch hairs (around .5 to 1.5 cm.). The hairs collect debris, and the plants can be killed because so little light reaches the leaves. The hairs are too tough for ramshorn snails or most fish to eat, and blackouts do not work. I once tried a 6 month blackout, and the Oedogonium was all still alive when the light was returned, and started growing immediately.
Cladophora is a close second. It is bushy, it sticks to the gravel, and, if not pulled out weekly, can cover the plants and stop their growth, probably mostly due to nutrient depletion. It also smells bad. It is tolerant of low light. To kill it with a blackout, you would probably have to have it in total darkness for a year or two.