SELECT TOP 1 completed, auditdate
FROM tblAudit_AuditSchedule
WHERE agentNumber = '54321'
ORDER BY auditdate DESC

select Max(auditDate) from tblAudit_AuditSchedule where agentNumber = '54321' 

select AgentNumber, Max(auditDate) from tblAudit_AuditSchedule group by AgentNumber

select completed, AgentNumber, Max(auditDate) from tblAudit_AuditSchedule group by Completed, AgentNumber

SELECT completed, auditdate, a.agentNumber
FROM tblAudit_AuditSchedule a
	INNER JOIN (
		SELECT agentNumber, MAX(auditdate) maxdate 
		FROM tblAudit_AuditSchedule 
		GROUP BY agentNumber
		) b ON a.agentNumber = b.agentNumber and a.auditdate = b.maxdate

-- or 
SELECT completed, auditdate, agentNumber
FROM tblAudit_AuditSchedule a
WHERE auditdate = (
	SELECT MAX(auditdate) 
	FROM tblAudit_AuditSchedule b 
	WHERE a.agentNumber = b.agentNumber
	)

-- or
;with LastAuditDatePerAgent AS (
		SELECT agentNumber, MAX(auditdate) maxdate 
		FROM tblAudit_AuditSchedule 
		GROUP BY agentNumber
	)
SELECT completed, auditdate, a.agentNumber
FROM tblAudit_AuditSchedule a
	INNER JOIN LastAuditDatePerAgent b ON a.agentNumber = b.agentNumber and a.auditdate = b.maxdate