Your title is a bit off, you mean 'news-server' not newsgroup.
But it basically comes down to three:
Giganews - very expensive, very solid. You tend to get what you pay for, but that doesn't mean that they don't sometimes run off the rails, so to speak.
Astraweb - very inexpensive, runs off those rails a lot more (skips and such), but has been getting better and better over the last couple of years.
Usenet-News - king of the third party providers. Very solid, if one goes with Astraweb they are the best for 'fills'
I'm sure everyone has a favorite from the tiers below these three, but it comes down to speed, connections, completions, and retention, and all three of these are within a couple of percent of each other.
FYI, only Astraweb and Usenet-News do 'block' accounts, 'pay as you go', with blocks that never get 'old', or 'roll off'. Giganews hasn't done blocks in at least 15 years, their accounts are more time/transfer rate based.
If you want to dip your toes in, a block account is the way to go.