Plenty of amazing and informative answers already. However, one option that hasn't been mentioned is square [brackets]!
I personally like this method in mediums or platforms that do not support italicizing, and there are plenty of such platforms, ironically ones that are even involved in heavy use of writing, like some text-roleplaying or play-by-post and collaborative fiction websites.
I do not know how the use of the asterisks, like
*so*, started, but I do know that they are used heavily in some text-roleplaying or collaborative writing communities, traditionally to represent actions of characters in the story, though, as opposed to dialogue. Moreover, the asterisks are used on countless websites and mediums to show italicized words after you press Enter or Send, including right here on Stack Exchange! So maybe that's why others use them so much, even when their words don't show up italicized. I personally don't cringe at the use of asterisks for emphasis, or don't find it unacceptable, but it's not my favorite way of emphasis.
And I don't like the use of underscores, like
_so_; this just looks wrong to me personally.
The use of the slashes, like
/so/, on the other hand, is much more easy on my eyes. However, I would still feel like something's not quite right.
So finally, square brackets are actually formally used to represent the author's additions in a quote. You can see this quite often in good newspapers when the journalist is quoting someone. For example, the article could say, "The burly mechanic said to me, 'The way we do things here [in the basement of his shop] is as follows...'". Or it could say, "The band leader showed me his electric guitar and referred to it as '[his] weapon of mass destruction'"; in this case, the author or journalist is clarifying that the word "his" was obviously not used by the band leader himself, but you, as a reader, will be able to guess that the band leader is quoted word for word otherwise, and he said "my" instead of "his".
So one can argue that we should not use square brackets this way so as not to confuse our readers about the purpose of the square brackets, and while I tend to agree with that line of thought, I also would personally consider my audience and what exactly I'm writing. And I would argue that when we are in a medium that does not support italicizing, like most chat programs today on the phone and desktop computers, we are most probably not going to confuse our "audience" when we refer to [the] thing we love most about, say, action movies. So yes, I'm personally happy using square brackets to emphasize the occasional word, and I find it the most elegant and easy-on-the-eyes way to achieve this in mediums that do not support italicizing.