Honestly, this depends entirely on what kind of mood you're in. Melody Carlson has so many different series and stand-alones targeting different ages. If you want her more contemporary, slightly edgier teen stuff, you gotta start with the 'Diary of a Teenage Girl' series—it kicked off her YA career. The order there is pretty straightforward: 'It's My Life', 'My Name Is Chloe', 'Sold Out', then 'Road Trip'. But she's also huge in the Christian fiction world with series like 'The Carter House Girls', which is a different vibe entirely, more about friendship and family drama. You can't really go wrong, but jumping between her teen dramas and her adult Christmas novellas might give you whiplash. I usually recommend picking a series and sticking with it for a bit.
I tried reading her Christmas books in July once and it felt weird, so maybe save those for the season. The 'TrueColors' series is also great for tackling tough issues, but those are more thematic than a continuous story, so order matters less.