As Karen says, italics only. No quotes.
Quotes indicate words actually spoken. (With the non-descriminatory caveat that they can involve non-verbal forms of speaking.) So, if all the communication is telepathic, then you would put them in quotes as it would be the default means of communication. But in any setting where what we would consider normal speech is the norm, thoughts should either be in italics, or not indicated at all.
The supporting logic for the latter choice is that leaving them as plain text reduces psychic distance. If you italicise them, you are calling out that they are thoughts, inserting an authorial layer between character and reader.