I have used USGS 7.5-minute topos in flat areas that have 5-foot contour intervals. Be aware that much of contours are just interpolation between known elevations. In other words, you can have two points, one 50 feet the other 100 feet elevation and simply draw 10 evenly spaced contours between these data points or simply draw 2 lines, one for 50 feet and the other for 100 feet. The map with the 10 contour lines is NO MORE accurate than the one with 2. However, even though no more accurate I find the terrain easier to "see" on a map when a few more contour lines are already drawn. USGS maps sometimes use dotted contour lines. These lines are just interpolation.

Most topo maps now are computer generated based on Digital Elevation Models (DEM). DEM's are a grid, say 30 meters apart, of elevation points. Most DEMS are based on satallite data. These points are "ground truthed" by many fewer surveyed elevation points. I have used DEMS that were flown by private companies contracted by coal mines to create monthly mining maps. We used these maps to calculate our mineral royalty payments. These maps had 2-foot contours and were quite accurate too. You will not find this kind of map available to the public.

In general most USGS maps have 40-foot contour intervals for mountainous terrain, 20-foot contour intervals for low hilly terrain and 10-foot for flat areas. Then there are those odd-ball metric maps (I hate them) with 20-meter contour intervals.

There are books out there that explain how contour maps are made. I find it very informative to read these. That way you do not think the contour maps are less or more than they really are.