Flat, wide, and long is the safest but the reuslting bow might not be a good shooter. I'd say that whatever design you choose for your first bow the chances are it won't turn out that great. It may, of course, turn out perfect but bowyering seems to be one of those skills that develop iteratively.
Maybe the best option is just to decide on a design you like either due to historical or aesthetic significance and go from there. The exceptions being recurves, sinew backed bows and hornbows. Recurves tend to have stability issues, sinew and horn bows requre skills other than tillering.
To conculde; my advice is to choose a design you like and then perfect it, every style is different and all can be tricky when making them for the first time. More experienced guys may have more helpful and informed advice.