MyBB - Mods - MyBB Search Engine Optimization
This one will make the links in your forums more attractive and it will get you more visibility in search engines.
MyBB - Mods - MyPlaza
Just something you and your users can have fun with. It's a good way to encourage users to post.
MyBB - Mods - Posts Required To PM
Prevent or at least help prevent spammers from spamming via PM.
MyBB - Mods - Forum Warning System for MyBB 1.2.x
MyBB doesn't have an infraction system like vB does. This MOD acts just like one though. It's really helpful when you need to enforce rules and take care of those problem members.
MyBB - Mods - Moderator Control Panel
This is a definite must. It allows your moderators to ban and do other things that aren't available to them and they normally wouldn't be able to.