With minimum defects and maximum functionality working
I've found 4.3.6 and 4.3.8 to be pretty stable. However, YMMV due to products, options, features etc. Every merchant is different and things like shipping options (etc) all affect stability.
4.3.8 is pretty stable.
Agree with 4.3.8
As a merchant 4.3.3 was also rather stable however new releases have incremental improvements and if you haven't had any major changes done to the core files an upgrade is worth the money.
Upgrading however is not always smooth but that can never be guaranteed fully.
Overall I am as a merchant very pleased to made the switch to CS Cart. I hope CS Cart will be here for years to come.