Is saying hao3 ch1i ba rude?
The most common way to express bon appétit in more formal situations is:

请慢用

请[qing3] = please

慢用[man4 yong4] = Enjoy your meal! / Bon appetit!

If you find this too formal you may take off the 'please' (请) and just say: 慢用.

Mind you this is most likely going to be said by a waiter/waitress in a restaurant or someone who is not joining the meal.

慢慢吃

is an option, but also usually said by someone who has finished eating before the rest of the party. 

慢

Although, 慢 does mean slow(ly) in general in phrases like 慢点 (take care/be careful) or 慢慢X (慢慢走(take care)、慢慢吃(enjoy!), etc.) it doesn't, literally, mean slow but to take your time.

好吃吧

好吃吧 is made up of two parts 好吃 + 吧 —or— tasty + (I) guess. So this phrase really means - (It's) good, I guess.