I personally found A+ Gallery Photos & Videos to be a perfect choice for Dropbox photos syncing if you use Android.
A+ Gallery offers everything a good photo gallery has, and it's better in basically all aspect than Google Photos and Android Carousel.
Google Photos would also be a fine option as long as you are willing to switch to Google Drive for your photos storage. Besides the cloud storage,
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it also provide lots of interesting features that's dependent on that you enable cloud backup.
For mobile use I'd still recommend A+ Gallery Photos & Videos over Google Photos.