Starting with the last question: do you expect to get any of the currently bloomable 6* soon, or would you gamble on one of your 6* to become bloomable anytime soon? (there's some theory that Viola Yukata is next on the list, but nothing is certain). If you feel you want to hold out and bloom another girl, then that's pretty understandable. But Cherry does improve quite a bit from blooming. Her stats go up by 15-20%, she will get a 10% ATK buff for the entire party, and against bosses, the ATK buff now counts for the entire party, and they get another 10% damage bonus as well.

Whether it's worth it to bloom a gold unit: well, for strength, things are roughly like: 6* Bloomed > 5* Bloomed > 6* Not Bloomed > 5* Not Bloomed.
So, if you have the resources to spare, it won't hurt to bloom a few of your favourite 5* girls. But do keep in mind that you may eventually replace them with bloomed 6* girls. But, we're also still 7 or 8 months away before we have 20 unique bloomed 6* girls to make a full unit, so 5* girls you bloom now will last you for a while.

Now, finally the first question: that's really personal. First choice is always your favourite girl. But from your collection, I'd say Alpinia, Anemone and Cherry are the ones benefitting most from HP and DEF ampies. Alpinia from taking less damage and a bigger healthpool, for her lifesteal to become even more effective, Anemone because of her counter ability, Cherry because she lowers the enemy's ATK, which is the first step of making her very tanky in encounters (second step is having more girls with that same passive in her squad, third step is raising her DEF to ridiculous levels).
Every girl benefits from ATK ampies, though Alpinia does get special mention on this one as well, since she has life steal, so benefits from ATK both offensively and defensively.