That's a very good question that I'm not going to answer directly. In part, because I shouldn't be making direct quasi-medical recommendations without having a doctor-patient relationship.

United States Pharmacopeia is a non-profit and ConsumerLab is a for profit company that both have good track records for reviewing supplements. For ConsumerLab you have to be a paid member to see their rankings.

Safety and purity appear to be good across the board with the products they have surveyed. But some products cost as much as 10x greater than others. Also, doses above 400 mg of L-theanine have not been tested for safety.

My own bias would be that if you trust or like a particular supplement manufacturer, to go with their product, and if not, then opting for the cheapest makes sense. Most of the research suggests that L-theanine benefits are likely to appear quickly, within days, on a given dose.