Well (over and above the standard manager) surely that depends on what you are running and what you have assigned to each manager?! Check the specialization rules of the managers and if there is a job included on a manager that you need to run then you can say that manager is pretty important.
You need to monitor preety much all the concurrent managers as they all play different role.
Most important concurrent managers you should monitor are ICM which takes care of all other managers.
Standard Manager - which takes care of most of the concurrent requests.
OPP- Which takes care of post processing of concurrent requests.
Conflict resolution Manager - which takes care of conflicting requests.
Workflow% managers- which takes care of Workflow.
For more information on other concurrent managers I would recommend you to read oracle documentation on concurrent managers and you will get better idea on each and every concurrent managers which are running in your environment.
I hope this answers your question.
Thanks for the info