Motoros are generally the recommended starter ray. They are a hardy ray and don't cost a fortune. Make sure and read the stickies to be sure you are ready to care for a ray.
Yup motoro gets my vote too, Just keep in mind you will get roughly a year out of an 18" wide tank roughly 2 years out of a 24" wide tank if you are starting with a small pup.
Could be more or less but plan on upgrading to at least a 36" wide if not 48" wide tank at some point
They are awesome fish and as long as you keep up on your tank cleaning / water changes they are not that difficult to care for
As far as smaller rays goes i agree with the above post, Hystrix would be what i would suggest. But make sure you look through some other I.D. threads and stickies though, alot of FAKE Hystrix out there be sold as True Hystrix.
i asked this same thing a while back and was told since i would be beginning to get a motoro not a hystrix because histrix are more sensitive and are sometimes harder to get to eat. but like they said, i was told to read the stickies thoroughly before even thinking about getting one. good luck