I believe that Donald Trump is the only Republican that can hope to defeat Hillary, in fact, I think he will not only beat her, but it'll be a humiliating loss for her majesty. Trump's careful to not venture to the right, as Cruz is, as in today's environment, I believe a hard right ideologue is unelectable. It'll be nearly impossible for Cruz to backtrack on any of his stances for the general, without coming off as a "flip flopper." The only thing that may be extreme about Trump is his immigration stance, but that is NOT arisen from an ideological perspective, it's all about security.
Now on why he, and only he, can defeat Hillary, is :
1. Hillary can't play the vagina card on Trump. She paid her female secretaries less then their male counterparts when she was a Senator, while Trump is an equal opportunity employer. Bill runs around on her constantly, and she hasn't taken him to task for it. Plus by rolling him out, it might show feminists that as a woman, she needs her man to make a case for her.
2. Hillary can't call Trump a warmonger. Trump is just waiting for this, so he can pounce on her about her not only voting for the Iraq war, but holding a long winded speech on the Senate floor urging her constituents to support it. Trump can then show how he opposed it from the get-go. Bam!
3. Hillary can't say he hates gays. Trump can show videos of her from her time as first lady and her failed Presidential bid in 08, where she opposed gay marriage. To my knowledge, Trump has never spoke about this issue.
4. Trump can portray her as a flip-flopper. In her 08 run, she enthusiastically supported the right to bear arms, ANY arms. Opposed to now how she wants restrictions severely undermining it.
5. Hillary will say he hates immigrants. Trump will just playback his speech where he bashed ILLEGAL immigrants. Actually, I think he's likely to increase legal immigration, as he has said.
Trump is for all intents and purposes, running a general election campaign. He has kept all social issues at a distance, as he knows that social issues are a drag on the ticket, and a loser for the general. On the Palin endorsement, I don't know how that will play out, but I think she'll be pretty much pushed to the sidelines come the general.