Ive been a sega fan all my life and I'd say Segas output of games can be rated the following way:
1. 1998-2001 - Best games during those years were made by Sega. They were the best software company in the world ((ans A+)
2. 1991-1994 - Segas golden days when they woudl release a B+ game every month and have solid core AAA franchises like Sonic and Phantasy Star (Rank A)
3. 2005-2009 - Segas internal studios are failing to achieve greatness but their publishing choices are quite good with games like Condemned, Bayonetta, Madworld etc...
4. 1995-1998 - Sega is somewhat struggling with the saturn but manages to release a handful of classics such as Nights and Panzer Dragoon. (Bank B)
5. 2002 - 2005 - Sega can't find themselves in the industry and are startin to milk their known franchises (Rank C)