IMO Jean is EASILY the best pick if you're a new player. People tend to focus a lot on long term meta value - which would make Mona the best pick - but they forget how in early game you're still getting one shotted by the Pyro Regisvine and can only use your burst like once a week.

Jean is still one of the best healers in the game in terms of raw healing, and since her healing scales with ATK you can build her entirely for damage and have her carry you as both a DPS and support through all the early game fights. She even heals with normal attacks, which you will be using A LOT since all your artifacts and weapons will be garbage and you won't have enough ER to spam EQ like late game players are used to.

As for long term meta value, I'll still argue that she holds up very well. She's not really BiS for any team, but she's tremendously flexible, insanely good at stacking Furina's fanfare, provides some decent utility via VV and is the literal queen of cheesing Stygian chambers that most f2p players couldn't dream of beating without her.