It's really early and things will obviously change between now and the first week in June, but Keith Law runs down his top 50 (plus 10 others) MLB draft prospects. I wouldn't pay strict adherence to the order of the ranks, but it's a useful introduction to names that you will begin hearing as the draft gets closer. 

Keep in mind that because of three unsigned picks plus all the compensation picks, the Mariners second pick is actually the 62nd overall. Between those two selections, I think the Rays draft twelve hundred times.*

*It's actually ten times. Still, ten picks in the top 60!

The article is behind the ESPN Insider paywall so I'll limit the posting to Law's top five:

Anthony Rendon, College RH 3B*
George Springer, College RH, OF
Gerrit Cole, College RH, P
Bubba Starling, HS RH, Athlete (committed to Nebraska for baseball and football)
Sonny Gray, College RH, P

*By the way, it's Rendon pronounced with a long 'o' so Ren-Doan, not Ren-Done or Ren-Den