It's almost certainly a US mission, as the US spends more on space than any nation, and depending on how you pay attention, NASA missions or spy satellites are the most likely candidates to be that mission. So let's take a look at those categories. Note that I'm looking for missions that are the most expensive by categories, often not explained, but...
- Curiosity- \$2.5 billion
- No space telescopes of any note launched.
- Parker Solar Probe- \$1.5 billion
- Rosetta- 1.5 billion Euros
- Tranquility ISS module- \$400 billion
- JUNO- \$1.1 billion
So for NASA, it seems the most expensive mission launched in the 2010s was Curiosity.
As for spy satellites, well, that's a bit trickier. Those spacecraft aren't known for certain what they cost, but there are estimates out there. Candidates include:
So I would guess that pretty much any KH-11 satellite, such as USAA-224, is the most expensive payload of the 2010s, with an estimated cost of $6.35 billion. Of course, all of the latter missions being classified makes it really difficult to estimate the price.
Lastly, the upcoming mission that will certainly be more expensive is the JWST, with a cost of around $10 billion.