Under the succession law at the time (the Third Succession Act and the will Henry VIII made in concurrance with the Act), the line of succession after Henry was:
- Edward VI
- Mary
- Elizabeth
- Frances Grey (nee Branden) and her descendants (Jane, Catherine, and Mary Grey)
- Eleanor Clifford (nee Branden) and her descendants (Margaret Clifford, Countess of Derby)
- Henry's remaining heirs general (either Mary, Queen of Scots or Henry Stuart, Lord Darnley)
Edward, Mary, and Elizabeth were explicitly listed in the third succession act, and the Greys, Cliffords, and Stuarts were named in Henry's will.
Edward wrote a will of his own (under the advice of the Duke of Northumberland), which (if valid) would have skipped Mary, Elizabeth, and Frances and gone straight to Jane Grey, and I think he planned to have Parliament amend the Third Succession Act to confirm it, but he died before he got the chance.
Legally, there's very little question that Elizabeth would be the rightful heir. Politically, it's possible Northumberland would still try to get Edward to skip Elizabeth in favor of Jane, but since Elizabeth was protestant, that removes a lot of the urgency, and makes it questionable whether Edward would go along with it even if Northumberland tried.
Mary Queen of Scots is probably the only plausible rival (she's a minor at this time, being raised in the French royal court, but her mother Mary of Guise was on the verge of displacing the Earl of Arran as regent of Scotland). She's the next plausible Catholic in the line of succession, and she'd be the heir but for the Third Succession Act (Elizabeth is legally a bastard, as Henry's marriage to Anne Bolyen was annulled before Anne was beheaded, and Stuarts are geneologically senior to the Brandon/Grey/Clifford branch of the family even though Henry's will elevated the latter about the former). The legal basis for Edward's will, that not even an act of Parliament can place a bastard in the line of succession, can be made at least as easily in favor of Mary Stuart as for Jane Grey.
Supposing Edward still signs a will purporting to disinherit Elizabeth in favor of Jane, I'd expect Jane to wind up with almost no support, even less than OTL: as a Protestant, Elizabeth is much less scary to the Protestant nobles than Mary was, so pretty much the only reason to back Jane over Elizabeth would be desire to support Northumberland in a transparant power grab. I'd expect Elizabeth to pick up a fair portion of Mary's OTL support, although some Catholics might try to install Mary Queen of Scots instead.