Do you know this is so hard to answer!
There are so many options for Shellac now that I find I do such a variety of finishes that there isn't a specific one or even two.
I guess brights are popular as the Summer is here but also funky patterns and Rockstar/Pearl.
I'm wearing tropix layered with Mother of Pearl at mo, purely by accident as I top coated using my glitter top coat by accident so then just added mop to sort it out. Bit to blingy for me but I've had several clients say they love it.