There really isn't a best answer to your question...how long is a piece of string? How big is the tank that will sit on said stand? Will you enclose the stand in a cabinet fashion? To be honest, your question is pretty arbitrary.
But.
I'd have to say anything but Balsa... It depends what type of look you are after. I've used finger jointed pine in 66x42 for my stands and it holds up perfectly, but i don't have anything over 90L. Also, being pre-primed clear grade, it painted up very nicely... You might want a rustic look, in which case rough sawn pine or hardwood would be best suited. If you are asking "What size timber is best?", then good ol' 4x2 is hard to beat, but you can use 6x2, 3x2 (as I have), 4x3... The list is endless.