AssaultCube offers the best balance IMO. It's 90s-style with the custom maps and all, but it's semi-realistic and not too fast or crazy. Game play is simple yet has a high skill cap. Graphics are pretty good. Public servers are all casual and unranked (there are also private ladder ones), but people still try their best. And it's free and low-resource.

I tried this web CS1.6 and was immediately greeted with "your skill level is too low for this server" :/