Best SSD for Asus Z97-A?

Solution
You cant go wrong with the Samsung 960 EVO. It is an NVME M.2 SSD and very fast, but your M.2 slot wont support PCIe 4x speed. You would have to get a M.2 PCIe adapter and put it in one of your PCIe slots for it to work at 4x speeds.

Honestly, it is really not worth the trouble. There is not much of a noticeable difference in performance from NVMe to a standard SATA SSD. I would go with the Samsung 850 EVO. It is probably the best selling SSD on the market. The Crucial MX300 is a cheaper option and it has good performance as well. I would go with one of those options.
You cant go wrong with the Samsung 960 EVO. It is an NVME M.2 SSD and very fast, but your M.2 slot wont support PCIe 4x speed. You would have to get a M.2 PCIe adapter and put it in one of your PCIe slots for it to work at 4x speeds.

Honestly, it is really not worth the trouble. There is not much of a noticeable difference in performance from NVMe to a standard SATA SSD. I would go with the Samsung 850 EVO. It is probably the best selling SSD on the market. The Crucial MX300 is a cheaper option and it has good performance as well. I would go with one of those options.
 
Solution
Define "best"
From a performance point of view, a Samsung 960 m.2 PRO would be best.
Primarily because it can use the 10gb/s transfer rate if the connection.
However, realize that the big benefit of a ssd is in the low latency that windows likes for what it does.
From that point of view, all ssd devices perform about the same.

Best from cable management point of view would be the M.2 device since it plugs directly into the motherboard. Downside of this is that it is a bit more awkward to change around the plugged in device.
At least this motherboard does not use the space under the graphics card like many others.

Best from reliability would be Samsung of whatever type. They have a 5 year warranty.

Larger ssd devices perform better and last longer.
240gb os ok, if that is what you think you need with room to spare.
If you can, buy a 500gb Samsung 850 EVO