What is a good 6x3 prop?

Actually one would have to look at all factors, not necessarily running at an RPM that is at the highest HP output will give the best flight. It all depends on the type of plane, desired performance, etc.

Granted the 6x3 prop may not place the static run Tee Dee .049 at its highest horsepower, when it is in the air dynamics change. Also, satisfactory flight is depended on the torque curve as well as the HP curve.

According to SceptreFlight Nov-62 Aeromodeller Cox Tee Dee .049 by R.H. Warring, the torque curve is relatively flat from 13,300 to 20,000 RPM peaking at 18,000 RPM, decreasing as one approaches maximum HP at 22,000 RPM. A Top Flite nylon 6x3 prop with 25% nitro turned at 18,400 RPM, at the peak of the torque. A 5.25x3 TF nylon turned 21,000 RPM toward its maximum HP.

SceptreFlight Dec-61 Model Aircraft Cox Tee Dee .049 though not stated appears to be written by Peter Chinn. It's observations are similar.

Dec-61 Model Aircraft wrote:Performance. The Tee-Dee .049 was very definitely much happier when allowed its head - in other words, when permitted to turn at least 17,000 r.p.m. This is not to say that the engine lacks power when loaded for lower speeds. One of the remarkable features of the .049's performance is its ability to turn comparatively large props (e.g. 7x4, 7x3) faster than any other engine of similar capacity. However, it is apt to become a trifle uneven and for r.p.m. to wander slightly on such props, whereas it smooths out at above 17,000 and is absolutely rock steady at speeds around the b.h.p. peaking speed of 21,000 r.p.m. The fastest we had the .049 turning was 23,000 and it seemed quite prepared to go on at this speed indefinitely. [....]

The actual torque developed by the Tee-Dee .049 was the highest yet recorded in this series for a 0.8 c.c. engine and reached a maximum of 8.1 oz. in. at around 14,000 r.p.m., equivalent to a brake mean effective pressure of approximately 64 lb./sq. in. - a figure which, a few years ago, would have seemed impossible with a Half-A glow engine. The decline of the torque curve was at an almost constant ratio up to 20,000 r.p.m. and resulted in the quite phenomenal b.h.p. of .145 at 21,000.

CL fliers take advantage of the torque curve, turning larger props at slower RPM and slowing engine speed down a little running a rich 4 cycle that breaks into 2 during stunts even though modern sport Schneurles are designed to provide more power at higher RPM's.

Figuring that the engine unloads once plane is up to full speed means an another 1,000 - 2,000 RPM to the static bench running figures, bringing the engine closer to its higher horsepower figure.

Thus as Bernie points out, the sales of the props with feedback from buyers tell a slightly different story. In the right setting, that 7x3, 7x4, 6x3 or 6x4 prop may be more than suitable on the Tee Dee .049 although it is not driving the engine at its maximum rated horsepower. Then other factors enter in such as fuels and glow heads.

Also of interest, according to SceptreFlight May-61 Aermodeller Cox Babe Bee & Golden Bee the 6x3 and 6x4 props run close to maximum HP, making them the right prop for these engines. I like them for my Black Widows.
Prop selection is perhaps more of an art than science.