Thought I would start this thread so as to keep questions together and avoid side-tracking/hijacking other posts.
As a starter.......
My own preference is for Asuswrt-Merlin which is commonly referred to as merlin's firmware.
Its basically the stock Asus firmware but has been tweaked to fix issues and improve it. The person that makes it has a good relationship with Asus and many of his tweaks get based back to Asus to use in their own updates.
The benefits of using this alternative firmware is that it keeps the same look and feel as the original firmware.
[about quote] "its primary goals are to enhance the existing firmware without bringing any radical changes, and to fix some of the known issues and limitations, while maintaining the same level of performance as the original firmware.
It is also well documented on his own website but there is also a lot of support available from the user community and by merlin.
I have two Asus routers. one as my main network (Asus firmware) and another for testing/projects/vpn, etc. (merlins firmware). For this reason I use merlins firmware as it looks close to the stock firmware and is easier to manage the two routers.
There are other firmware options available and are possibly better but I have found I need to do a lot more reading to get things to work. eg with merlin's software its like a Toyota Corolla. Its familiar to drive and just works. The alternatives are more often like a Tesla where I have to do a lot more reading to get things to work. It might be better/faster and have a "ludicrous" mode but sometimes there's way too much involved in getting it to work.
In other posts @michaelmurfy has mentioned advanced tomato. I do not have any experience with this so can't comment but it does look really good based on screenshots. A quick look shows that it is based off Tomato by Shibby which is another personal favourite but can be a bit too "Tesla" at times.
Links
Asuswrt-Merlin (merlins website and downloads)
Small Net Builder user community and support from merlin
merlin on GitHub