Mistborn (Eras 1 & 2)

For me, the Mistborn novels showcase Brandon Sanderson's primary strengths better than his other Cosmere series. Those strengths being Magic systems/usage and speed of production.

Mistborn has, for me, one of the best magic systems ever created in Allomancy. The magic is rooted in logic, with distinct rules and consequences. The suspension of disbelief required is actually quite a small bridge to cross, and the payoff is fantastic. Allomancy is cool. The powers allow people to accomplish many of the typical childhood superpower fantasies, but without copying and pasting those abilities in. All of the Allomantic powers are well thought out, and the way characters use their abilities feel realistic. Consequences of burning tin aren't just better hearing and seeing for spying, but also light and sound sensitivity. Steel pushing and iron pulling require careful consideration of mass and trajectory. The abilities aren't treated like mythic superpowers, but practical abilities that enhance a subset of the population. Hemalurgy and Feruchemy are similarly impressive and well thought out, though way less explained or understood (especially in Era 1).

Most importantly, though, Sanderson's practical and scientific magic is put to use in practical ways to actually cleverly solve problems! So many magic systems are wasted. They may sound fun or look cool, but many authors still resort to Deus Ex Machina. Sanderson's solutions to conflict are clever and real. And when he started combining abilities within individuals/teams, the potential uses and solutions got even better. We particularly see this in Era 2, with the conclusion to Alloy of Law being my favorite use.

As for the books... Mistborn, the actual first novel in the series, is a solid epic fantasy with excellent magic systems and a couple compelling characters. The plot and the action are well within the normal bounds for Fantasy, the magic and it's use being what elevate above its peers.

Well of Ascension has similar attributes to book 1, but the ending completely turns your expectations on their head and not only sets up the conclusion of the trilogy, but made you completely reconsider everything you had already read to that point.

Hero of Ages was a fitting conclusion, expanding upon Scadrial and the powers of its world. HoA though is where we start to truly get introduced to the broader Cosmere. Shards and the power/responsibility/attributes that come with them are now fully in play. If you like the idea of interconnected series via godlike channels, then this is where things start to pick up. For me, the Cosmere is actually a drag on the main series. I loved Alloy of Law and appreciated Wax and Wayne for what they are. The more Sazed/Harmony got involved, and the more the rest of the Cosmere intruded, the less invested I became. This is part of what is raising the hairs on my neck with Stormlight Archive. I like the books for the books. The series for the series, but I'm not sold on the Cosmere itself.