Ctrl++ is probably the most understandable. To make it:
I think Emacs formatting would confuse a lot of Ubuntu desktop users.
Also, unless you're specifically referring to the +-only key on the number pad, it's probably a good idea to write Ctrl+Shift++. (Edit: See Marcel's point below.)
For keyboard shortcuts in a question title, it looks like the Ctrl++ form is most commonly used. While this form has more potential for ambiguity, it's probably best to clutter titles as little as possible and let your question clarify the meaning.
On a side note, if you ever need to type inequality signs you can use the HTML entities
> to type < and >, respectively.
And one more thought: It should go without saying, but, for everyone reading this, please don't abuse the <kbd> tag.