0 kts speed is best to being silent but worst for tactical reason
There is no simple answer - your main role is to be exactly on time into your patrol area. So if you have 1 hour and 15 nautical miles to your partol area, best speed will be....
..as always, your maximum tactical speed - no lower than 15 kts.
you should go faster than minimum speed because you can find something "unwelcome" on sonar or other sensor :P
Before transit you should check layer depth. If you can transit under layer - you should go fast, but under layer.