What would be the most efficient light for a singl...

For a 80cm x 80cm x 160cm tent I use a 400W HPS light but in Summer it's very hot, for Winter I don't have any problem Good grows 👌

As others already said, I would go for some high quality LED light. Quantum Boards are very good. For a single plant, this one might be sufficient: https://www.sanlight.com/en/luminaires/sanlight-m30/

I would go with a Led Board . Atm is anew gneration on the market , they are passive cooled. super silent and great output of light ( lumen .. needing less wattage than the generation before. If you are going to grow more often. Its absolutely worth to go with a quantum board. I will let you a link in the commensts. I would either go with that MEIZHI CREE COB 300W Led Grow Light Veg Flower or with that ...... Spider Farmer 1000w LED Grow Light Quantum Board Samsung Diodes Full Spectrum, that should be enough for 1-2 Plants, Ive bought a few days before the SF-2000 ( spiderfarmer)... and iam very happy with the lights, they are as bright as the light on a sunny day on the Beach

would say a cob light as they are super energy efficient or even the led light .CFL take a lot to set up and you need a few of them to do anything good .if your looking for easy growth and not much heat I run a 300watt viraspectra led and she does good for my girls in vegetation or flowering cycle and don't cost much or effect my energy bills .. that's my two cents or the cobs they look really good just a bit more expensive , the best would be these quantum boards very thin light and good on energy consumption from what I have seen so far..