I've got one '15 and one '16 ticket and so far my lineup consists of 6* Lavender, 5* Hare's Tail Grass (Halloween) (Nutaku Lagurus), 5* Himalayan Creeping Saxifrage (Nutaku Bergenia), 5* Pinkladies, and the new event wossname girl.
Congrats for 6* Lavender. Considering she is the one and only 6* in that 18' stone gacha (it's a special gacha for the new bloomed artworks for 6* Lavender, 5* Lady's Sorrel and 5* Clover and thoses 3 are the only 6* and 5* in it), you are lucky.

For the 15' stone, you must go for Cymbidium. She is the most useful girl here wit her unique x2 skill activation turn 1.
For the 16' stone, you must pick Pink Miko (aka Dianthus (Miko) on Nutaku). She is the most powerful 5* in the game, better than some 6* when she is max equipement slots. Even now, she is still helping me on the hardests maps.