I will give you a quick overview of what I have found. No invitations are needed for the indexers I use. AFAIK we can name names here, right?
NZBPlanet has been good for me, and I bought lifetime there years ago. However, as time went on I noticed some holes in their coverage--mainly for older stuff. I still use it as my primary, though. I am not sure if they still sell the lifetime tier, but registration is open so you can take a look. I am not sure what the free tier gets you though, it isn't listed in their tiers page. If you are interested primarily in current stuff, I feel like the lifetime membership still ain't bad. Registration appears to be open.
I found that DrunkenSlug filled in a lot of the gaps that I found on NZBPlanet. I liked it enough that I recently bought a year. The free tier at 5 downloads per day is not generous, but it can definitely help if you are in a jam. They also have open registration, at least right now.
ABNZB is also in my rotation, but only at the free tier, which is also 5 downloads. (I think I am a "legacy" free member with 25 downloads.) My impression is that its coverage is similar to NZBPlanet but once in a while it helps me find something that I don't find at Planet or Slug.
Lastly, the totally free and primitive looking binsearch.info is worth a bookmark as a site of last resort. It's bailed me out before.
If I was starting over from scratch I might do Slug instead of Planet.
We're all looking for different stuff and it's hard to be definitive about coverage, but these are my impressions.
For a server I have been very happy with Eweka. I don't even have a block account elsewhere for fills. Once in a blue moon there is something I can't get at all, I just roll with it. I already have more Linux ISOs than I can use. :)