The last game I "S" ranked was Borderlands and with the recent 'news' of a possible fourth add on..that may soon be in jeopardy seeing as I don't own the game( Gamefly helped with that one, had it out for 5 1/2 months) but anyways.. I am literally 4 achievements away from "S" ranking Fallout 3...I've got to level a neutral character from 1 to 30 and get my lvl 25 Bad character to 30 and i will have them all!! 
Also I am 5 achievements away from "S" ranking Splinter Cell Conviction, with them all being fairly challenging.
Lastly,I have 1 achievement left in Limbo and it is the hardest one at beating the game in 1 sitting dying 5 or less times