I feel that Tim Cook's crowning achievement would be in building a formidable ecosystem around the iPhone. One that has greatly increase its stickiness and led to more and more people buying iPhones every year, even going so far as to continue stealing significant number of "switchers" from Android.
Perhaps by themselves, products like AirPods or TV+ don't really seem ground-breaking, but put them all together and take a step back and it's quite an immersive ecosystem Apple has built for itself over the years. I know that in 2016 alone, I bought AirPods, Apple Watch, iPad Pro and Apple Pencil and these are all products released under Tim Cook's watch.
And I think the decision to focus on the iPhone took courage, especially when it likely meant neglecting the Mac at one point in time (or at least, Apple didn't appear to be particularly enthusiastic about it). Tim Cook correctly realised that there were more iPhone users with Macs than there were Mac users with iPhones, and so decided to prioritise the former. Even when it meant angering the latter.
The strategy clearly appears to be paying off handsomely. Many people have the iPhone has their sole Apple product, meaning there is still a lot of potential to upsell them on additional Apple products and services. Which in turn means that there is still a lot of room for Apple to continue growing in the future.
I think this is where Tim Cook deserves credit. He has taken the company handed to him and brought it to ever greater heights, when it could have easily gone south in anyone else's hands. Remember how many people here were pushing for Apple to acquire Netflix, or invest in smart speakers, or even release a folding phone, or a hundred other "Apple should do X" statements that turned out to be utterly boneheaded in hindsight? It would have all too easy to make one wrong bet after another if you lost sight of what made Apple unique and just listened to the rumour mills.
I can personally think of no better person than Tim Cook to be running Apple right now, looking at the state of the other tech companies like Google, Intel, or worse, even Tesla.