Hello Ben,
I have been using Mixpanel and Amplitude and both of them are good solutions in order to track events on your product; events, funnel analysis, retention and so on. The most important thing is that you set up properly the taxonomy and naming convention of your events' structure. I don't have anything against Segment I tried once but I didn't work with that tool but it for sure is a good one based on other workmates' opinions. ¡Important! When tracking distribution channels, take into consideration the attribution channel and all the touches that the users do in all the channels before your conversions. Some of them are good to open a path and other ones good to close.
Besides that, for A/B testing there are several options in the market but VWO is a good one that with a lot of features to get good insights into the A/B testing. Another one could be Optimizely or Google Optimize. ¡Important! Some teams instead run A/B testing because sometimes that could be a technological issue, they run cohort tests, where basically they change features on the product and they analyse this performance with the new users and compare. That could have another issue like seasonality but it is another option.
Overview, I recommend Amplitude and VWO but just to make simple the answer, because Mixpanel, Segment and the other options that the other users wrote here are good too.
All the best, and let me know how your experience was with all these tools. :)
Cheers,