Easy to Keep Pipefish
I've keep pipefish, seahorses and even shrimpfish (similar in care of syngnathids) in the past. And they only eat frozen, no pellets or other dry foods.
I've kept dragonface pipefish, banded pipefish, blue striped pipefish and janss' pipefish. I currently have the latter two.
The dragonface never ate frozen foods, there mouths are extremely small but it ate the microfauna in the tank. I wouldn't really recommend them unless you have lots of pods.
Banded pipefish are hit or miss to me. Some eat quickly while others are slower to learn, however I believe if they are healthy and strong enough they will eventually eat.
The easiest in my experience are janss' and blue striped pipefish. With blue striped pipefish being easier between the two. They are open water swimmers and usually learn to recognize frozen foods. Especially if you start with small foods such as nutramar ova, roe or bbs. Then you can wean them onto mysis. My current blue striped only eat nutramar ova. While my janss' eat mysis. Both fed twice a day.
Overall the "easiest" pipefish are still difficult to care for. Be sure that they are healthy before buying and ask if they eat before hand. This will help them become accustomed to aquarium life and eventually eating frozen foods.