Depends a lot on the plane but here are my thoughts for most "40 sized" 3d planes

O.S. .46 AX - excellent 2 stroke for run of the mill 3d'ing- 12 x 4 prop, low maintenance,

O.S. .55 AX - Never used one personally for 3d but it does have more power for sport flying than the 46. same transitions etc - most likely a 13 x 4 prop.

Saito .82a - The absolute best of three - excellent power, excellent transition, turns larger diameter props and the lowest fuel consumption of the 3. - Prop - 14 x 6 or 15 x 4 depending on the plane.

I use 15% nitro on all of my engines with 17% Coolpower and 3% castor.

All of the engines listed are good quality, all are fuel efficient, all are low maintenance once set up properly - it depends on your budget and the weight of the plane.