1st gen: I never played in this gen
2nd gen: I only played the 2600 so i don't have a personal opinion here.
3rd gen: Master System was more powerful in almost every aspect, but the NES had the titles because of its restrictions, besides, Ninty did the hard work restoring gaming in NA, if they pulled out that, they had to win that gen...
4th gen: MegaDrive and SNES had similar power, Megadrive was capable of faster gameplay (because of clock speed, Sega milked that with sports games and mostly with Sonic), but SNES was capable of Mode 7 (F Zero, Mario Kart, Turtles in Time, Final Fantasy VI, the great Chronno Trigger, StarFox, etc.), and it had enhancement chips designed for specific games (the Super FX for StarFox or CX4 for Megaman X2 and 3, SA-1 for Super Mario RPG, etc.), what defined the era was the games, especially when Nintendo decided to release Mortal Kombat II uncensored (after the friendly Mortal Kombat debacle), the SNES ended with the kiddy system myth (at least in that gen), and they won the battle, and no happy with that they released Donkey Kong Country with 3D graphics, like saying "we are all that and a bag of chips"...
5th gen: The Saturn had power, especially for 2D, but it was really phreaking hard to develop for (devs complaining this gen, they don't know squad about hard development unless they made a Saturn game), quadrilaterals instead of triangles (in basic geometry, piece of cake, in electronic graphics, a pain in the ass), parallel processing with lots of restrictions (1994 people) and 0 documentation... N64 had the best 3D engine, but the cartridges resulted in few content, lack of killer sound and less textures, besides the high production costs... If the Saturn and N64 were the only ones in the war, some devs would take sides and Ninty would have the victory with an average but still good Sega as 2nd, but the Playstation took away most of the devs that Sega was supposed to get with the easier development, and attracted a lot of the Ninty ones with the CD, it offered both worlds in an average but strong system, the result was the lost of most of 3rd party support that Nintendo had, and the desperation of Sega that gained a really bad reputation, they shut down the Saturn and moved on to the Dreamcast but it was already too late...
6th gen: The Dreamcast was Sega's return, but they never recovered from the Saturn and they failed creating a strong base before the PS2 release, one down, one entering... The Playstation 2 entered with virtually no competition, it got the games, it had the Emotion Engine hype, and the PS1 fanbase... Nintendo instead came with a purple cube system with 1.5 gb discs, this was enough to pull a 64, but they were not aware of the Xbox, it was the most powerful of the 3 in almost every area because it was the 1st one near to what a PC was offering at the time, but the games already were on Sony's side, still Microsoft gained popularity with the Dead or Alive franchise and shooters, it was the beginning of the Halo franchise, this was enough to beat the Gamecube...
So in my opinion, in the 3rd and the 6th gen the best systems didn't win, and the 7th gen is looking like that, but the 4th and 5th gen were won by the best systems overall...