I use all three - Picatinny, MLOK and Keymod.

For metal forends, I want a Picatinny on the top or the option to mount one. I prefer Keymod for the sides and underside.

For plastic forends, I prefer MLOK in all positions.

I don't care for built in Picatinny rails in all positions. Bare Picatinny rails are not comfortable. Adding covers is a solution, but some covers make the diameter even bigger than the obese Glacier Guard style forends or Magpul MOE.

Looking at accessories, as a generality, my impression is that there are more MLOK accessories than Keymod and generally the MLOK accessories are a bit less expensive.

For metal forends, almost everyone makes them with slots in the 3, 6 & 9 positions, and 12 is either more slots or a Picatinny. But if you want a forend with slots in all clock positions, that may dictate the mounting system you use, instead of it being the other way around.