70 watt DE halide.
But it really depends on what you are planning to keep. A 70 watt halide would be unnecessary for just soft corals and LPS corals. For those a 96 watt PC fixture would work nicely.
Like viper said If you arn't planning on keeping SPS corals than go with the 20" 96watt Coralife Quad 50/50 powercompact fixture. I have one on my 10 gallon and love it.