The M4 platform has its detractors, but as long as it’s kept relatively clean and well lubricated, it is as reliable as any firearm extant, and as for accuracy and ergonomics, the M4 is hard to beat. If the platform has any real weakness, it is the 5.56mm cartridge. Several cartridges have come down the pike recently that were intended to replace the 5.56, but the .300 AAC Blackout is, in my opinion, the most viable option.
The .300 AAC Blackout was designed to give the M4 platform increased power and penetration on intermediate barriers with minimal recoil while retaining the 30-round magazine capacity. Its designers also intended for it to provide excellent terminal performance and accuracy when run suppressed with subsonic or standard ammunition. After working with various Blackout guns for the last year, I have to say AAC met those design goals.
I currently own two Blackout rifles, a 16-inch gun from Ambush Firearms, which is the hunting division of Daniel Defense, and a tricked-out 16-inch carbine from Houston Armory. Both have collapsible stocks and are threaded for suppressors. Both have 1:8-inch twist rates and Geissele triggers.