SAP Basis Important T-Codes

SMLG-Logon Groups
ST03N-Work load Analysis -Record Average Response Time
ST01/ST05-SAP Trace
SM21-System Log

SP01-Spool Requst Overview
ST02-Buffer Statistics
SM37-Background Job Log
SM51-Sap  Servers
SM66/SM50-Global Process Monitor
ST22-System Dumps
SM58-t-RFC
SM13-Update monitor