Plywood works fine as long as you buy good plywood and a good "fish safe" waterproof paint or epoxy to seal it.For building a large tank considering,cost,strength and ease to work with, nothing beats plywood
Glass also works ,if you have the tools to cut it.
Acrylic works good,but the tools to cut it are expensive.
It depends on how you want to display it ,how big you want it and personal preferance on which works better.
You can look through the posts in this forum for some good ideas,since we've had a few discussions about building large tanks lately.
Some decient sites that should help you out are :
Geothermal Aquaculture Research Foundation, Inc.
http://www.cichlidfish.com/Adam/diytanks.html
Aquarium Thickness Calculation
How to Calculate the Glass Thickness for your Aquarium
www.mr4000.com