I'd been using some super cheap LED tail-light that I got from...I think it was rakuten.com. It was very similar to this, and about the same price: eForCity Bicycle Rear Lamp, 9 LED - Rakuten.com
I think mine only had 5 LEDs, 2 of them blue (which is probably illegal in all US states). Anyway, I had no complaints with it; it was bright enough, and a single set of batteries would last me at least one season.
Unfortunately, my rack-top bag blocks seat-post mounted lights, so I clipped it onto the tail-light loop on the back of the bag. Last week, it went flying off that loop when I hit a bump, and it's gone forever.
Then I switched to the tail-light that came in this set: SHARK 300 - USB RECHARGEABLE BIKE LIGHT ? CycleTorch
It's tiny, nicely bright, and super easy to mount to a seat-post, but not to the tail-light loop on my rack-top bag or panniers. It had relatively limited side visibility, but I also wear LED Defiant-brand arm bands that my mom got for me a while back for side visibility. (These things: DEFIANT Mini LED Flashlights (Safety Arm Bands (2-Pack)) - - Amazon.com)
Unfortunately, that tail-light has been lost.
I replaced it with the cheapest tail-light my LBS had, some Planet Bike model. It's adequate, but I don't trust it's hard plastic clip on the tail-light loops on my bags; I'm afraid I'll lose it when I hit a bump.
Also, my Cycle Torch headlight has sold me on USB recharging for bike lights, although that's not a deal-breaker.

So, after all that, what tail-lights do you guys like for mounting onto the loops sewn onto many rack bags and panniers? I'm willing to break the $25 budget for the right light.