I have a question for you out there who have or have had lots of different scopes. Which scope gathers light the best?
As some people already have mentioned, a combination of appropriate exit pupil and maximum lens size is the basic combination. Glass quality and coating is another factor and there are huge differences between brands and product lines.
First of all, in most european countries hunting isn't restricted by sunset or dawn so higher-end european manufacturers have made low light performance a priority. The obvious choice for hunting at dusk, dawn and moonlight has been Zeiss Diavari 3-12x56 or Diatal 8x56, both offering exit pupil of 7mm at 8x magnification. They have dominated magazine tests at least since the 80's and while they still are great scopes for low light hunting, Swarovski has managed to equal or even beat them with their Z6i 2.5-15x56. They are the top choices if low light performance is a must and even though some manufacturers like Schmidt & Bender have come close, they still have an advantage. A few years ago one of the more reputable magazine tests surprised me, because even Nightforce got its butt soundly kicked by Swarovski and Zeiss. Leupold, Bushnell Elite, Burris and many other quality scopes were so far behind that it wasn't even funny anymore.
If you don't have $2k or so to burn, there are a couple of manufacturers with strong bias towards low light performance but without Zeiss/Swaro price tag, namely Meopta and IOR/Valdada. I've used Meopta Meopro 4-12x50 for a few years now and while it doesn't compare to Diavari, I've made quite a few clean shots with it in nothing but moonlight. In low light it's very noticeably better than 4.5-14x50 VX3 and has one of the best BFTB ratios of mid-priced scopes. I've also used Valdada 4-14x56, which is even better, except for its $1000+ street price. Still great value.
There's one low light scope that I really, really would like to have. It's the discontinued Zeiss Conquest 3-12x56. Optically it's very close (if not identical) to older Classic Diavari, it was priced at less than $800 and even though illuminated reticle wasn't available, it was the low light scope bargain of all time. I had a chance to compare it to a Victory Diavari, side by side, when sun was setting and it took a conscious effort to find a meaningful difference between the two. Massive value. I'd hazard a guess that a major factor for it to be discontinued was that it (almost) could rival scopes twice its price in low light performance.