DBA : Find the most expensive queries in SQL Server

SELECT TOP 10 SUBSTRING(qt.TEXT, (qs.statement_start_offset/2)+1,

((CASE qs.statement_end_offset
WHEN -1 THEN DATALENGTH(qt.TEXT)
ELSE qs.statement_end_offset
END - qs.statement_start_offset)/2)+1),
qs.execution_count,
qs.total_logical_reads, qs.last_logical_reads,
qs.total_logical_writes, qs.last_logical_writes,
qs.total_worker_time,
qs.last_worker_time,
qs.total_elapsed_time/1000000 total_elapsed_time_in_S,
qs.last_elapsed_time/1000000 last_elapsed_time_in_S,
qs.last_execution_time,
qp.query_plan
FROM sys.dm_exec_query_stats qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) qt
CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) qp
ORDER BY qs.total_logical_reads DESC -- logical reads
-- ORDER BY qs.total_logical_writes DESC -- logical writes
-- ORDER BY qs.total_worker_time DESC -- CPU time

–Queries taking longest elapsed time:
SELECT TOP 10
qs.total_elapsed_time / qs.execution_count / 1000000.0 AS average_seconds,
qs.total_elapsed_time / 1000000.0 AS total_seconds,
qs.execution_count,
SUBSTRING (qt.text,qs.statement_start_offset/2,
(CASE WHEN qs.statement_end_offset = -1
THEN LEN(CONVERT(NVARCHAR(MAX), qt.text)) * 2
ELSE qs.statement_end_offset END – qs.statement_start_offset)/2) AS individual_query,
o.name AS object_name,
DB_NAME(qt.dbid) AS database_name
FROM sys.dm_exec_query_stats qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) as qt
LEFT OUTER JOIN sys.objects o ON qt.objectid = o.object_id
where qt.dbid = DB_ID()
ORDER BY average_seconds DESC;

–Queries doing most I/O:

SELECT TOP 10
(total_logical_reads + total_logical_writes) / qs.execution_count AS average_IO,
(total_logical_reads + total_logical_writes) AS total_IO,
qs.execution_count AS execution_count,
SUBSTRING (qt.text,qs.statement_start_offset/2,
(CASE WHEN qs.statement_end_offset = -1
THEN LEN(CONVERT(NVARCHAR(MAX), qt.text)) * 2
ELSE qs.statement_end_offset END – qs.statement_start_offset)/2) AS indivudual_query,
o.name AS object_name,
DB_NAME(qt.dbid) AS database_name
FROM sys.dm_exec_query_stats qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) as qt
LEFT OUTER JOIN sys.objects o ON qt.objectid = o.object_id
where qt.dbid = DB_ID()
ORDER BY average_IO DESC;

The above queries were taken from
http://blog.sqlauthority.com/2010/05/14/sql-server-find-most-expensive-queries-using-dmv/