How to find which plugin is taking maximum resources

Best bet is usually to check error_log.

Have you got a plugin throwing a lot of messages?
If your log file is big, you can delete it (it will get recreated if more errors appear).

Another thing to look at is: http://gtmetrix.com/
It’s mostly client side, so it possibly wont be too useful for narrowing down on a plugin… it may give you other ways to reduce the size of (and thus, simultaneous requests upon) the site.

What is the site?