I have played Commandos series, GTA series and Hitman series.
No doubt Commandos Men of Courage was the best among the series. I still have it installed on my PC.
GTA San Andreas was best, haven't played GTA V yet, but GTA IV was not very interesting.
Hitman series is uncomparable, you can complete one mission in hundreds of different unique 'silent' ways and it 'never' gets boring. I liked the Hitman Silent Assassin and Blood Money most. Hitman Absolution was good, but I did not like it very much.
Red Alert 2 Yuri's Revenge and C&C Generals Zero Hour were great too.
Though I have played lots of games in past, but these were the memorable ones.