How do i find out the most active database?
Capture this:

Select  instance_name As Database_Name,
        cntr_value As TranSec
From    sys.dm_os_performance_counters where counter_name = 'Transactions/sec'                  
And     instance_name &lt;&gt; '_Total'
Order By instance_name


Then run it again at the end of the day. Subtract one set of values from the other. That will give you the most transactions.

You can look for a similar counter for logins/sec but it may be misleading depending on how the app connects to the database and if it keeps the connection open or not.

Another good way is by the disk IO but this can be misleading too as a lot can show up and be caused by backups/maintenance.

Select  Db_Name(mf.database_id) As Database_Name,
        Sum(a.num_of_reads) As Reads,
        Sum(a.num_of_bytes_read) As Bytes_Read,
        Sum(a.num_of_writes) As Writes,
        Sum(a.num_of_bytes_written) As Bytes_Written
From    sys.master_files mf
Cross Apply sys.dm_io_virtual_file_stats(mf.database_id, mf.file_id) a
Group By Db_Name(mf.database_id)
Order By 1


Again you'd run it in the morning and again in the afternoon and subtract to compare.