Monaural recordings image best!Seriously, I test imaging with old jazz mono recordings, they can reveal a lot when setting up speakers.
For stereo soudstaging, Reference Recordings are great, especially if you like classical. Respighi Pines of Rome is great, it won a grammy for best classical recording a decade ago. Provides a very large, wide, deep and tall soundstage. Strong LF ambience cues, if the playback has low distortion bass (large drivers.)