The most reliable cpanel alternatives I have used that are free are Ispcomfig and Virtualmin.
Others that have free offerings are
Centos Web Panel
I am currently using Virtualmin, CWP, and Cyberpanel. Virtualmin is extremely complicated to use but by far the most feature rich for GUI use, CWP and Cyberpanel have much nicer interfaces (particularly for client/user dashboard).
Cyberpanel uses Openlightspeed web server (it's the only one that natively does). For single use, Cyberpanel is a great alternative to whm cpanel. For webhosting use for clients shared hosting im fairly convinced that the paid version may be a necessary choice (due to the need to protect your server from client abuse by assigning user quota and limits in webhosting)
If you want top notch support and reliability...definitely ISPConfig. However setting up a server to use it is a marathon...it's frustratingly slow to configure anything on Ispconfig...but once setup and fine tuned, a very reliable panel. Has no file manager!
If you need file manager, maturity and stability, Virtualmin 100% (virtualmin docs and support aren't the best though...not terrible, but forums support for "virtualmin-gpl" free version could be a lot better than it is.) Virtualmin is a massive learning curve...its a very powerful control panel and its dashboard needs a street directory to find things!
Centos Web Panel is a great looking control panel but absolutely full of little stupid mistakes that cause bugs in it...things like WordPress script installer stuffs up adding password to wp-config.php...hence a brand new user account WordPress install throws a "cannot connect to database" error.)
If you did choose CWP, best option is the monthly lease...it's cheap as at like $5 per month for the main server panel "pro version"...so across 30 clients...That's miniscule At like 60cents per user(oops math terrible this morning...just over 16 cents)
If you want free, stable, maturity and reliability, functionality, and provide shared hosting for clients, however have clients who are willing to put up with its somewhat disjointed interface design...out of all the free panels, virtualmin is "by far" the safest option. I cannot understate how big of a learning curve it is though. Also, if you start messing around with virtualmin defaults without 100% knowing what you are doing...you will destroy it and have to wipe the server and start again. I have been using it for 2 years and have done that many times.