Cyber Sleuth and Time Stranger are probably the most accessible games for someone that isn't really into Digimon for sure
For my money though the most interesting game in the franchise is the original Digimon World, with its direct sequel Digimon World Next Order also being interesting for similar reasons, but not quite having the same weird charm to me. They might seem hard to start with, but that's mostly because they're such unique titles, but they're the most Digimon of all the Digimon games. The idea is they're a fusion of the original Virtual Pet toys and an RPG. The visual design is absolutely on point and my favorite in the entirety of Digimon, and the fundamental gameplay loop is oddly cyclical. You train your Digimon up until it's powerful enough to clear the next area, and then you quickly race around File Island trying to clear as many objectives as possible and power up the town until your Digimon expires and respawns as a baby, at which point you repeat the cycle using your upgraded town to make it even stronger than the last time. It's a weird, wonderful, and unique game with a lot of flaws, but it's an experience you'll never forget.
I just meant more that it's a lot of extra content in addition to whichever game they choose to play first.
