Either Adobe Target or VWO. Both have their upsides sure, but both are also an absolute quagmire of terrible design decisions that aren't consistent in the slightest, and that are prone to break an A/B test if you even look at them wrong.
In Target's case, this means stuff like 'install a browser extension when our software doesn't work, so it can load the code that browser security settings will often block', and 'log in via an Incognito window if the editor doesn't work properly, since some setting is now incompatible with your current API version and the interface to disable said setting breaks along with the entire editor'.
In Target's case, this means stuff like 'install a browser extension when our software doesn't work, so it can load the code that browser security settings will often block', and 'log in via an Incognito window if the editor doesn't work properly, since some setting is now incompatible with your current API version and the interface to disable said setting breaks along with the entire editor'.