The best ones, then, tend to be the ones that have been around long enough, or had enough foresight to have covered various corner cases.
The other issue is that most of them have "per user / per month" pricing, which doesn't work for many use cases. Things like, for example, an applicant tracking app. If you use the functionality to a high degree, every job applicant is another end user license.
Two of the better ones: http://quickbase.com & http://ragic.com
Other ones I've seen: http://caspio.com, http://knack.com, http://fusio.com, http://trackvia.com/, http://podio.com, https://www.zoho.com/creator/, http://airtable.com, https://getzenbase.com/, http://www.zenginehq.com/
One of the more promising ones was DabbleDB. Twitter acquihired them and shut the tool down. It had a very smooth UI: https://www.youtube.com/watch?v=MCVj5RZOqwY