CASE WHEN max(c.Client_Changed)> max(m.tlog_date) 
     THEN CASE WHEN max(c.Client_Changed) > max(t.transactionpurchasedate) 
               THEN max(c.Client_Changed) 
               ELSE max(t.transactionpurchasedate) 
          END  
     ELSE CASE WHEN max(m.tlog_date) > max(t.transactionpurchasedate) 
               THEN max(m.tlog_date) 
               ELSE max(t.transactionpurchasedate) 
          END   
END

CASE WHEN max(COALESCE(c.Client_Changed,'17530101'))> max(COALESCE(m.tlog_date,'17530101')) 
     THEN CASE WHEN max(COALESCE(c.Client_Changed,'17530101')) > max(COALESCE(t.transactionpurchasedate,'17530101')) 
               THEN max(c.Client_Changed) 
               ELSE max(t.transactionpurchasedate) 
          END  
     ELSE CASE WHEN max(COALESCE(m.tlog_date,'17530101')) > max(COALESCE(t.transactionpurchasedate,'17530101')) 
               THEN max(m.tlog_date) 
               ELSE max(t.transactionpurchasedate) 
          END   
END