これ好き

I like this
Not including a honoriffic like -san or -chan when naming someone else is considered quite informal, and doing so outside of a personal context (e.g. friends, family) is considered quite rude. Also, note that Aunn still refers to Reimu as Reimu-san, a level of politeness she might not actually need to maintain for a housemate, even as she leaves it off for Sanae.