I run a Cygolite Hotshot 150 as my primary "SEE ME" light in the daytime. It's more than bright enough to be seen in direct sunlight. Driver WILL NOT miss you. I keep that on a 1 blink per 1.5 seconds flash and it's plenty. (And that that blink rate the battery last just short of forever) The limitation is that the beam can only be seen straight back. It's not alerting anyone to the sides that I'm there. But the driver's directly behind me at teh target audience.

Slightly higher than that I have a Cygolight Hotrod 50. It's still bright enough to see seen in daylight, but not from as far back. I keep that on a faster blink setting. 1 per every half second or so. It's must more eye catching, but not as bright. This light has a much wider viewing angle. So it offers me decent lateral notification as well. (Note, it does not mount horizontally like in the image below natively. I had to get creative)

Basically the 150 grabs a driver's attention from a long way back and the 50 keeps their attention as they are close with the faster blink rate.

For night riding the 150 is uneeded. The Hotrod 50 is more than enough to announce my presence on the road.The 150 is more than overkill in the dark. It's downright blinding.

Get a front light too. Go brighter. I run a Cygolite (I like Cygolite...can you tell?) Dash 460 in the front to make driver's coming the opposite direction see me. Some people go even brighter in the front.