The Spurs made the best trade, but I don't think it makes them serious contenders, yet. This was a great move but it's likely only the beginning.
The Lakers got an elite scorer while sacrificing an elite defender. Their move involved the biggest names but I don't think it solves the Lakers' biggest problem which is depth. The thing is, if the Lakers fail to reach the Finals, they have to find assets to build around Luka (possibly without being able to trade Lebron since he's a FA)...so how is this really any better for Luka than his situation was in Dallas?
The Mavericks might actually have improved the most from this in the short term, but the damage to the brand and fanbase is massive. Davis increases the Mavericks' floor but there's still plenty of competition for them in the West.