I believe Sam's awful romances thing began with The First Commandment.

Oh, true. I tend to not count him because he doesn't recur (thank God).

I've never thought about "Enigma" as an entry point, but it's a great one. The opening of the episode is just perfect. The alien planet and situation is unusual and then we're immediately thrown a mystery when Omoc says not to help them when they clearly need help!