When it was released on January 14th 2003, SimCity 4 had its problems. Its huge cities would chug on even decent PCs, for one, and its traffic simulation seemed outright broken.
Twenty years later - thanks to faster PCs, the Rush Hour expansion, and a huge modding community - SimCity 4 is the best of all SimCity games. If what you care about is simulation, scale, variety, and the beauty of urban sprawl, it's also the best citybuilder.
Setting aside fantasy and medieval variants, the template laid down by the SimCity series remains the expectation for most citybuilders. As the mayor of a fledgling town, you zone commercial, residential and industrial districts, place down amenities and services to satisfy residents, and attempt to balance the budget as your town begins to take on a life of its own. Then, when you've constructed a sprawling metropolis and have grown weary, you trigger some natural (or unnatural) disasters and start over.
SimCity 4 did nothing to change this formula but continued its logical progression. It has 3D terrain and crisp, clear sprite-art buildings, which are one of the reasons it still looks great today. It lets you build enormous cities, and then trade resources with the neighbouring city you built previously. It has a day/night cycle, so you can see your skyscrapers twinkling in the dark. You can plop individual Sims into your city - The Sims 1 released in 2000 - and then track their lives as they commute from your polluted suburbs to their awful job, then move out of town or die.
Perhaps most importantly, SimCity 4 included the BAT, the Building Architect Tool, which enabled modders to create their own buildings and place them in the game. Custom buildings, maps and mods have greatly extended the life of SimCity 4 and at the time of writing there are over 21,000 files available from community site Simtropolis, including several new uploads in the past 24 hours.
Mods are why I'm confident calling SimCity 4 the best citybuilder of all time. More specifically, it's thanks to the Network Addon Mod. NAM set out in 2004 to fix SimCity 4's traffic issues, and as of its last update in September 2022 has expanded to include new UI tools, road types, light rail and much more. Given that every modern-day citybuilder inevitably boils down to tinkering with road networks and mitigating traffic jams, NAM is essential.
To address the skyscraper in the room: I love Cities: Skylines and if you're new to this genre, Paradox's SimCity milkshake-drinker is where you should start. If you have been playing Cities: Skylines and you're tired of it, however, or if you don't want to cough up for the next piece of DLC, then you should graduate to SimCity 4. The SimCity 4 Deluxe Edition (including the Rush Hour expansion, also released 2003) is on Steam and GOG. Given that 2013's SimCity was a bomb, and Maxis's original studio in Emeryville closed in 2015, we may never see its like again.