There is a wide range in how RPGs can be played. For TSR era D&D there it has a lot of in built mechanical flexibility. White Wolf games like WoD or Exalted adds a layer of dramatic flexibility at the expense of in-built heroics, which works well for a dark modern setting.

I really like a lot of games for different reasons. WW games, particularly Wraith, are some of the more interesting to run. Due to the higher reliance on player creativity and inter-character interactions. I really enjoy Wraith's shadow system for creating interactions between players for character flaws.

Paranoia is perhaps one of the most interesting GM experiences because it encourages so many deviations from standard gamemastering; railroading, PvP, splitting the party, killing PCs, ... . Still it works so well.