Well.......by THAT logic.....
In all blunt honesty, looking back at past generations:
1. Atari 2600 was not the most powerful of it's gen, but it sold the most and WAS the best console.
2. NES was not the most powerful of it's gen, but it sold the most and WAS the best console, by a country mile.
3. SNES actually was the most powerful of it's gen, unless you count the Neo Geo home unit, which is cheating because that wasn't a home console, it was a home arcade unit. But otherwise, SNES did sell the best of it's gen, and while Genesis had some GREAT games, I think overall most people consider the SNES to be the best system of it's gen.
4. The PS1 was by far the best selling, and certainly not the most powerful. Saturn was a great system that Sega abandoned far too early, and it had some great games. N64 was the most powerful by far, and had some amazing games, some of the very best of that gen (Mario, Zelda, Mario Kart, Star Fox, Goldeneye, Banjo, Turok, Perfect Dark, etc.), but was held back by the cartridge format, which caused many third parties to make games for PS1 instead. PS1 is widely regarded as the best console of it's gen.
5. The PS2 was by far, again, the best selling console of it's gen, AND it was weaker than GC and Xbox, though not THE weakest, as Dreamcast came before. Xbox had a sensation in Halo, and GC had some truly great games like Smash Bros. Melee, Metroid Prime, RE4, and Pikmin. But again, PS2 had the most third party exclusives, and in general is regarded as the best console of that gen.
6. Getting around to your point, Wii WAS the weakest of it's gen, and the highest selling. However, while the PS3 and 360 certainly had a lot of great games, most of which were third party games that Wii simply did not receive, and both had strong exclusives. The Wii also had a ton of great games, such as Mario Galaxy, Smash Bros. Brawl, MK Wii, Monster Hunter Tri, No More Heroes, Red Steel 2, NSMB Wii, DKCR, Metroid Prime Trilogy, Xenoblade Chronicles, and many more. It is, I think, far more up for debate in that gen which system was "the best", but it was a unique generation in that ALL three systems sold really well, and ALL three had a wealth of good games. And I say that having never been an Xbox fan.
7. Now this gen. It's not over, and still too early to tell, but thus far, PS4 is the most powerful, and the highest selling, and it does seem to be gradually building up a decent exclusives catalog. The Xbone is lagging, and has yet to receive too many "must have" titles. Wii U is certainly a sales disappointment, but it does have a bunch of very strong exclusives, including Xenoblade Chronicles X which just came out. In fact, arguably the two biggest surprises/hits of 2015, Splatoon and Mario Maker, are Wii U titles. But I think, barring something crazy and unforseen, PS4 is going to "win" this generation.
So....I dunno. History tends to show that the best selling consoles, on average, did also have the biggest and most varied game libraries, and are generally regarded as the best. Wii was a strange case in that it was by far the best selling, and DID actually have a shitload of games come out on it, but many key "AAA" third party releases skipped it.
We'll have to see what happens in the future.