My 50th game is from a very famous series, which was once great and which has now gone to crap.
I think one particular (earlier) entry in the series will feature heavily in these lists, and I love that game myself, but it didn't make my top 50. This one, however, just about did. It was the second last good game in the series to date, and I do not see the series improving in the near future.
Splinter Cell. Prince of Persia.