I you wanna say it pleasantly you should say :Piękna dziewczyna or śliczna dziewczyna You can say 'laska' BUT it's not always perceived positively, it's rather used in teenage slang and some girls...
Bella Ragazza (this implies Miss or Young Woman) Bella Donna is Beautiful woman but is also the name- as one word, of an addictive drug, sodium atropine, once uses as eye makeup by Italian girls,...