TeamCity - REST API - Last successful build

Adding a count of 1 should work:

/httpAuth/app/rest/builds/?locator=buildType:BUILDTYPE,status:success,count:1

You can get the id of the last successful build by executing the following request:

/httpAuth/app/rest/buildTypes/BUILDTYPE/builds/status:success/id

Then you can query the build itself:

/httpAuth/app/rest/builds/?locator=buildType:BUILDTYPE,id:BUILD_ID