Signum Pro polys, Tecnifibre polys, and L-Tec polys are good at maintaining tension. Downside is they are fairly expensive. You need to try them.
SigPro Tornado and Hyperion are excellent.
Tecnifibre BlackCode is excellent.
L-Tec 0S is excellent.
I use all 17G polys. 18G loses tension faster in general. 16G might be better for tension retention but it will normally be a bit stiffer too.
Some like hi-tension (> 52 lbs) poly but I encourage you to try 52 lbs or less. Even if it is not causing pain now, it may in the long term. Maybe 54 Lbs if you are playing an OS racket.