Bushnell TRS-25 is the best for under 100. Ridiculous battery life for the price (3k hours), tough, reliable, holds zero even mounted on my saiga 12 gas tube shooting slugs. I currently use it on my suppressed Ruger MK3. I accidentally left it turned on for several weeks in a row and it was still fine. You would have to leave it on for a few months before needing to replace batteries. I've still yet to go through the first battery and I've been using mine for years now. I turn it on when I go to use the gun, turn it off before it goes back in the safe. It's a great alternative to the 500-600 dollar aimpoint micros. Honestly, the battery life on this beats the crap out of most of the 400+ dollar optics out there.
Aimpoint PRO is the best for under 500. 3 year (always on) battery life, rugged, great glass, even NVG compatibility. I use this on my home defense gun because you know it's always going to be there when you need it. And it's cheap for what it does. Having this sight is what keeps me from buying the 800+ dollar optics. This thing offers almost every feature you could ever want except magnification and fancy reticles, at a price that beats the entire high-end market.
My feeling on C-More, Eotech, Vortex, Tasco, etc is that they all have crap battery life compared to the TRS-25 and you should definitely consider the bushnell first. It wouldn't be as big a deal if they could work without illumination (like the leupolds, acogs and vortex PSTs), but sub 1000 hour battery life on a 400 dollar optic is outrageous. If you can spend the 400 bucks, just get the Aimpoint PRO.