My opinion:
1st gen - PONG and dedicated consoles.
1.5 gen - Fairchild Channel F and RCA Studio II. Channel F was first programmable cartridges, and RCA Studio II was the only system that was a total zero - nothing good about it.
2nd gen - Atari 2600/Odyssey 2/Intellivision. The consoles that launched around 1977-1980 and reached their peaks in the early 1980s. Atari being the main driver of this generation.
2.5 gen - Colecovision/Atari 5200/Vectrex generation. Launched around 1982 and were more advanced than the previous generation. I would go so far as to split this into a different generation, only caveat is that it didn't kill off the 2nd generation.
3rd gen - This is easy. NES-dominated, with Master System and 7800. Launched 1985-1986.
4th gen - Sega Genesis (and add-ons), SNES, TG16. The 1989-1991 launch generation.
4.5 gen - The "fake" 5th gen. Atari Jaguar, 3DO, CD-i, etc. Launched circa 1993-1994.
5th gen - The classic N64 vs PSX vs Saturn battle. Launched 1995-1996.
6th - 8th gens are all clear cut, no transitional.