Bettas like to sit and rest on their plants so any with flat or crown-like leaves would be nice. You can go for real or fake but if you choose fake go for silk instead of plastic as rough bits on plastic can snag their fins
Plants which they can hide in are favourite. Try to avoid plants with jagged leaves which they could catch their fins on. I have anubias hastifolia in my tank and my betta loves to lay on the leaves on his side.