An indirect is probably the most efficient. But, that means the boiler needs to run all year. Depends somewhat on how efficient the boiler is. One designed with minimal internal volume and able to handle cooler return water temps would be more efficient than one designed to maintain a large quantity of water hot all year. The gas boiler I have only fires when heat is called for, or to keep it from freezing. Don't know if any oil-fired boilers work this way, if yours does, then it is a better candidate.
There's some heated discussion about tankless heaters. Somewhat depends on where you live and how cold your incoming water gets in the winter. Where I live, I'm not impressed. Other's experience may vary. They are expensive, an ddo require maintenance to maintain efficiency.
It has been suggested here to put an electric in series with the indirect. It would allow you to use it alone in the summer, and wouldn't need to run much during the winter, if at all. It would also increase the total volume available.