The LIRR puts out a ridership book each year where you can look and see how crowded your train is. If you go by the LIRR's way of calculating standees, they will say that all Babylon Branch trains have standees. However, if you adjust the formula to remove the middle seat from the equation (which few people ever sit in), you get much more accurate data from which you can decide which train would be the best to to take. You can view the adjusted load data here:
A More Realistic Look at LIRR Standee Counts (you will probably need
the timetable open in another tab to line up the trains).
Keep in mind that in the morning, the closer you are to where the train starts its run (the origin) the better chance of you getting a seat. So at Merrick, you would have the best chance of getting a seat on a train that has Wantagh or Merrick as its origin station.
Let me know if you need any help interpreting the table/finding which trains have the most seats available.