Frankly, I prefer all methods even though I have terrible luck in all three.
I guess to a lot of people it all comes down to what is fair (aka, what makes me the least salty/butthurt?)
Raffles are the most fair, in the sense they are drawings at random. However, I didn't like how the limit was set at 5 on the last sale, for example. Lower limits will allow for more ticket winners. People who enter your drawings will be more apt to choose what they truly want, instead of what they could resell/trade immediately upon arrival. Raffles are also great because you keep them open for 24 hours, easily giving everyone a shot.
I pretty much can't stand flash email sales, however a lot of people enjoy them. These are exclusive to basically anyone who is actively looking/has a notification on the Official Sale Thread at GH. Users essentially have a low amount of time to prepare or to even know there is a sale going on. A lot of users are basically at the mercy of their network when it comes down to properly timing these emails as well. Then you have some users might have an app or something that sends out an email at a specific time, which increases their chances (heard that at one point, no idea if people actually use this method). It seems like a lot of variables/factors can determine a winner here. I don't want to say the most skilled/apt mostly benefit, but there is some luck as well.
You could have an email sale that lasted 24 hours. You could establish a time where winners could be chosen over the course of X times a day. No one could win more than once. However, this would be more fair since it gave more people reasonable time frames to participate and you didn't leave a large chunk of users with nothing because they are in another part of the world. Basically just giving people more time and chances.
Group buys are obviously good. A lot of people got in, and there was a reasonable limit to what you could purchase. The problems arise when you have to deal with user inquiries at the beginning because of Stripe or a site problem. Overall, I think GBs are more than fair.
As I said, Raffles/GBs = super fair. However I think you could work on something that allows email sales to become a viable option for more users.
A lot of the complaints/concerns I mentioned throughout my post, are not exclusively my own, just what I might have seen as well from others.
TL;DR Basically, I would keep doing a mixture of all 3 - but with some tweaks.