As implied above, you want the prop positioned so you have maximum speed for the flywheel magnet as it passes the coil. That will give the best chance of starting.

For engines where magneto's etc aren't an issue (glow, electronic ignitions etc) my limited testing [testing the engines I have - mix of glow and some gas] placing the prop in-line with the piston/rod at TDC yields maximum revs. I'm sure some will dispute this but it works for me. My theorizing is that a single cylinder engine will never have a perfect balance. Adding a prop gives another plane of imbalance to the engine. It's better to have all those imbalances line up on a common plane or...theoretically...you now have two imbalances which could lead to vibration/harmonics that detract from engine performance.

I see variances of 300rpm on most glow engines depending on whether the prop is aligned with TDC or not. Always the TDC comes out with the higher peak RPM.