We can argue about "biggest" all day long but UTF-16 is a huge design failure because it made a huge chunk of the lower Unicode space unusable, thereby making better encodings like UTF-8 that could easily represent those code points less efficient. This layer-violating hack should have made it clear that UTF-16 was a bad idea from the start.

Then there is also the issue that technically there is no such thing as UTF-16, instead you need to distinguish UTF-16LE and UTF-16BE. Even though approximately no one uses the latter we still can't ignore it and have to prepend documents and strings with byte order markers (another wasted pair of code points for the sake of an encoding issue) which mean you can't even trivially concatenate them anymore.

Meanwhile UTF-8 is backwards compatible with ASCII, byte order independent, has tons of useful properties and didn't require any Unicode code point assignments to achieve that.

The only reason we have UTF-16 is because early adopters of Unicode bet on UCS-2 and were too cheap to correct their mistake properly when it became clear that two bytes wasn't going to be enough. It's a dirty hack to cover up a mistake that should have never existed.