IMO one that you have time for and can easily afford= foods,light bulbs, salts misc equipment and monthly electric bill. If you buy a tank to big can afford it or have enough time for it you will lo'se intrest and quit. I have a low maintace automated 120g. 4'x2x2' I would like a 60"x36"w x 24 tall someday. But if I were a buzy person who moved alot a 50g would be nice IMO something wider than tall that doesn't take up to much room. Just depends on your lifestyle and wallet. But the best tank is one that doesn't hog all your time and$$