Marcelo,
I would recommend using two Action Results, one to keep track of the largest number and another to keep track of the RecordID for its record.
Create the Action Result to keep track of the largest number and make it equal to 0 (create this before your Loop Through Grid action). In your Loop Through Grid, use a Conditional Statement to compare the value of hours (for the current record) with that Action Result. If it's larger than your Action Result, store that value of hours to that Action Result and store that record's RecordID to a second Action Result. By the end of the loop, your first AR should contain the highest number of hours from your grid, and your second AR should contain that record's RecordID.
Hope this helps.
- Justin