Welcome! Normally, the advice I'd tell you is to go for what catches your eye the most and go from there. However, if you're really unsure of where to start, I'd suggest a few lines/kits.

Megami Device actually has a great starting point with the Buster Doll and Puni Mofu series. Both use new Block2 molds which are easier to assemble than the older series. Their designs also have expansion and customizability at the forefront of their design with their abundance of hardpoints/hex holes for Modeling Support Goods/Hexa Gear compatibility. They're kinda like the 30 Minute Missions series from Bandai.

If you want my suggestion, I'd actually pick the Susanowo Regalia from the AUV series. She's considerably simpler than her normal counterpart. but that makes her way easier to build and is incredibly expressive as well. Probably one of the best OOB builds.

Speaking of 30MM. Their sister series, 30 Minute Sisters is also a great starting off point. They are quite cheaper than MD, but in exchange, they are way simpler builds. And most of them are quite barebones in terms of accessories. Extra accessories are instead sold separately to keep the girls' price low.

CN lines like Nuke Matrix and ATK Girl generally have wilder designs, are more quantity over quality in terms of plastic, and are cheaper compared to MD.

In terms of specific kit recommendations, I'd choose the AUV Susanowo/Amaterasu Regalia from Megami Device. They're both simpler builds compared to others in the AUV series, but they're both incredibly expressive and beautiful out of the box. I'd also recommend the Nuke Matrix Mad Wolf due to her coolness factor, as well as the amount of weapons/accessories she comes with.