-chan vs no honorific?

Hello! My question is which would be considered more casual/friendly (or rude to wrongly use): using surname-chan or surname with no honorific? I know surname-kun/san would be considered more polite than surname alone, but I was unsure with -chan due to it being an honorific used for endearment.

Everything can be rude when used in the wrong context.

Firas is right but in general, calling someone with no suffix at all ("yobisute") is the less polite way of calling someone and you wouldn't do it unless the person you're talking to or about is same age or younger than you AND you are very close to them.
So as a rule of thumb -chan is more polite but when in doubt just use -san. Better to be a bit too polite than a little rude.

Use -san unless they are little kids or tell you otherwise. For example some teenage girls insist on being called -chan.

Also I think you're confused about the meanings of -kun and -chan. There is no difference between the two except that -kun is used for young boys and -chan is used for young girls. They are both gendered honorifics. Obviously -chan can be used to infantalise young women the same way people refer to women as 'girls' in many Western countries.

-chan can be used for boys too, and sometimes some elderly teachers or superiors will call all their students or young staff -kun even when they are girls. So no, they are not STRICTLY gendered.

But yes, the best way is always to start by not yobisute'ing anyone and sticking -san to their name. You can always assess and adjust later.

It seems that people answering so far have first names or nicknames in mind. But the OP is asking about surname usage.

Let's say you're talking about a male named Suzuki Taro.

Suzuki: Absolutely Rude. But some boss-y people use this term, and some people do get away with being boss-y.

Suzuki-chan: You'd sound like an idiot or a kind of an easy guy. But some people do get away with sounding like an idiot or an easy guy.

Suzuki-kun: Normal if Suzuki-kun is a youth or clearly much younger than you. If he's older than you, this would sound rude.

Suzuki-san: The safest way you can address a person.

Of course, anything goes if you're very close with this person. But it's still safer to say, "Can I call you so-and-so?" before using that term.