It really depends. Do you want something that is easy to move or something that will stay in one place and last forever?
When I build my rack later this year, I will be using 1" white oak. This wood is nice and heavy and will last for years to come. However, it is also fairly dense and hard to work without proper tools, it can also get a bit pricey.
If you want something that is a little cheaper and easier to work, you can use pine. It is softer and can't take as much of a load, but if you just plan on using tubs or small tanks even, it should hold up nicely.
You could use douglas fir even.
Word of caution, if you do use a soft wood like pine or fir, make sure it is kiln dried and make double sure you seal every inch of it (with polyurethane). Not only will sealing it make it last longer, it will also keep you leos from getting respiratory problems.
Unless you build a sturdy frame, I would not recommend plywood.