Apple do sell mesh wireless routers. They provide no help or support though and they don't work in the same way as Apple routers used to do.. which is good and bad. NOTE.. Apple stopped router division 2.5 years ago.. so that was the end of that.
Mesh systems come in various flavours.
It is almost impossible to answer the general question.. what's the best one to buy?
There are many factors.
State what are your specifications.. i.e. what you are trying to achieve?
I have 5 to replace because they hiccup to often.
What models do you have and how are they setup?
If some are express used for airplay.. they can continue to be used with a mesh system.. since nothing replaces the Express directly. If only used for wireless extend they are no longer required. Gen2 express sell for good price so well worth the effort to put them on eBay etc.
Do you use a Time Capsule for Time Machine backup? This is important because no mesh system will replace that functionality but again you might be able to bridge a TC and use it JUST for backup. At least in the short term.. long term it needs replacing with a NAS or better wireless router or local backup only.
What is make and model of your main modem? Is it also a router? Note some mesh setups will have issues with this since the main mesh unit must take over the router duties.
Are you currently using ethernet to connect your Airports? Is ethernet even possible into the future?
wifi mesh will never beat ethernet.. so again that will make choice of mesh different if you are prepared to spend on wired ethernet.
NOTE>. If you own the house.. properly installed ethernet to a few strategic points in the house is expensive but will last the life of the building. The result is more reliable and fast consistent performance so well worth consideration.
What sort of budget are you looking at?
Is the home large or small? Solid brick and concrete or timber with hollow drywalls? Multi-storey or single storey.
You may not need mesh at all in a small home.. especially with modern AC or AX wireless routers far and away exceeding the performance of apple routers which at best were mediocre. Hence needing many extenders which then cause more issues. A single decent router plus a single AP in another location (e.g. second floor) and connected by ethernet.
NOTE.. if you currently have issues wireless extend could be a major part of the problem ethernet would fix.
What about the number of wireless clients in the house and how well you expect roaming/handoff to work??
The better mesh systems do a far better job of handoff so you can walk around the house than old Apple Extend wireless system.
Edit.. beaten by Bob.. but I have included some questions which will help the discussion.