Tbh I always preferred the slide phones. But if you specifically want opinions on the flip phones...
Needs to be able to open one handed with your thumb. Some models are too sleek with no ridge to pry up with your thumb.
Needs to be durable. A lot of cheap flip phones start having screens and buttons coming off after a while because of slamming the phone shut all the time and being made of cheap parts.
I preferred a phone with T9 texting, but maybe you can just download that these days.
Vibration might be a factor for you. Some phones vibrate really loudly while others are more subtle. When I was in high school I wanted a vibration I could feel but not hear if it was in my pocket or my hand. Many modern smart phones have much more aggressive vibration than the old flip phones. Not a problem for me anymore now that I'm an adult.