With only 7 days, in my opinion:
1) You'll be going for same-day or same-night lays, so
2) You'll be going for tourist girls, preferably alone and bored, and
3) You'll be going for girls from a culture where same-day or same-night lays are more of an option.
All of these point to major cities in Scandinavia or Iceland in summer as your best options in Europe. Krakow during the summer would be the best option of those you listed.
For Poland, and especially Ukraine, you need at least two weeks to follow a strategy involving any dates.
1) You'll be going for same-day or same-night lays, so
2) You'll be going for tourist girls, preferably alone and bored, and
3) You'll be going for girls from a culture where same-day or same-night lays are more of an option.
All of these point to major cities in Scandinavia or Iceland in summer as your best options in Europe. Krakow during the summer would be the best option of those you listed.
For Poland, and especially Ukraine, you need at least two weeks to follow a strategy involving any dates.