I want to design a forum. what is best frontend framework between bootstrap and materialize.
I would definitely go with bootstrap. It´s easy to get started, well documented, supported, responsive, mobile first...
If you want to read more to compare both frameworks I found a good discussion here
Also have a look to the twitter bootstrap components so you can get familiar with them. It has a lot of features out of the box and there are really good marketplaces out there where you can buy amazing bootstrap templates ready to be used.
Hope it helps!