Which improvement is the best to choose? I'm not completely sure exactly what they all do. The options are as follows: Longevity, Immunity, Resilience, & Health. What do you guys think?

Depends on how long and how trouble-free you want to keep your pet I should imagine.....long life, free from illness?Also depends on whether he/she died young or from ilness or from an overdose! The choice, as they say, is yours!