If you want something you can grow from a seed in a single pot and measure height and leaf area and that sort of thing, it's tough to beat beans or peas of some kind. If you want a plant that will just grow and grow and grow, get some bamboo -- it's the champion.

If you want to be able to measure the amount of 'biomass' that is produced in a certain amount of time, you could get some grass or sod, take sample clippings, and weigh them.

The best advice I can give, though, is to just go down to the garden store, find somebody who's worked there for years and years, and describe to them what kind of project you're thinking about and what kind of plant you want, and I'm sure they'll be able to fix you up.