I don't think I am going to vote in this one, because each decade has its own portion of being incomplete when it comes to absolute and total greatness.
The 1980s saw Nintendo creating the trifecta of Super Mario, The Legend of Zelda and Metroid along the launch of the NES that was home to quite a few legendary third party IPs. But this is 1985 onwards, so the first six years of this decade weren't anywhere close to that amazing.
The 1990s had the refinement of NES IPs on the SNES as well as proper portable gaming on the GB/GBC - as opposed to Game&Watch with its very primitive games - but also the Nintendo 64 with its tradeoff of clunkiness for the realisation of 3D gaming. The N64 is also when the breadth of Nintendo's home console library fell apart and the vast majority of true standout-titles began to be first party games.
The 2000s developed handheld gaming further with two great consoles: The GBA and the DS. But the home console side continued to suffer: First with the GC where some odd design choices hurt Nintendo's biggest IPs, then the Wii which couldn't get third parties back on board in earnest. Not Nintendo's fault, but it just ended up the way it did despite all the potential.
The 2010s are actually my choice for the worst decade, because that one is easy. Both the 3DS and Wii U were painful in their own ways and Switch came way too late in this timeframe to offset the miserable years.
The 2020s are flawless so far, but we are only four years in. Switch has restored Nintendo's classic and beloved setup of a game library, so it's the comeback of the NES/SNES era, plus it has almost all the greats that their 3D era has brought to the table.
Overall, Nintendo has always made great games. At times they've made some questionable choices for one or the other IP of theirs, but usually they had something else on the plate that wasn't there before or not as good before. They've had a lot more hits than misses. Even during their darkest era (3DS and Wii U) they've managed to have plenty of bright spots, even though they didn't result in more console sales; but sales aren't the topic here, so...
Legend11 correctly predicted that GTA IV will outsell Super Smash Bros. Brawl. I was wrong.
