These are what I normally use. I can't remember where I got them from, but they work a treat. This one's for longest-running queries:
SELECT TOP 100
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;
...and this one gives a list of queries using the most I/O:
SELECT TOP 100
(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 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_IO DESC;
Hope these are of some help!