carefullest

ˈkɛːfʊl
ˈkɛːf(ə)l

adjective

making sure of avoiding potential danger, mishap, or harm; cautious.

I begged him to be more careful

cautious,heedful,alert,aware,attentive,watchful,vigilant,wary,on guard,chary,circumspect,prudent,mindful,guarded,unhurried,measured,deliberate,leery,cagey