My sole strong preference is that I don't think I'd use single vs double quotes in the same story to distinguish thought vs speech. For the rest, it really depends on context; I prefer to keep the direct quoting of thoughts to a minimum, favoring indirect quoting/general narration. If you are making a point of quoting a thought directly, my favored form is italics -- but if you're doing a great deal of it, italics can get irritating/difficult to read.
I would never use ' or " as to me that implies speech. I generally use italics if the thought is as if the character is speaking (in their head to themselves) but not if it's the narrator speaking (ie, describing what the character is thinking). Examples which may or may not illustrate this:
As if the character is speaking (in their head to themselves):
Anderson, he thinks. <-- internal speech to self
Narrator speaking/describing the character thinking:
Twenty years is extreme for a distribution charge, he thinks, even for this sector. <-- description of thoughts, not speech
no subject
Date: 2013-05-22 06:36 pm (UTC)no subject
Date: 2013-05-22 06:47 pm (UTC)no subject
Date: 2013-05-23 01:25 am (UTC)no subject
Date: 2013-05-23 02:25 am (UTC)no subject
Date: 2013-05-23 11:32 am (UTC)As if the character is speaking (in their head to themselves):
Anderson, he thinks. <-- internal speech to self
Narrator speaking/describing the character thinking:
Twenty years is extreme for a distribution charge, he thinks, even for this sector. <-- description of thoughts, not speech
no subject
Date: 2013-05-30 03:43 pm (UTC)i'm so-so; you?
no subject
Date: 2013-05-30 03:44 pm (UTC)