Good idea to get a clearer picture of what channels you need and how you will use them. What's "best" for one person might not be best for the next. For most people that ask me about data, they actually have no interest in looking at physics data on a lap top after they drive. Most only want accurate lap times without needing a transponder, shift lights and some extra gauges on the dash. For those folks, you shop for sensor inputs and display. For those that are actually going to download the data and look at driving line, minimum corner speeds, shift points etc, you are really buying software, not hardware.
The most advanced D/A systems have powerful but less intuitive GUI's (graphical user interface) or software. I'm a Racepak dealer and love the stuff for it's great blend of affordability, scalability and flexibility. We run it in our race cars, reading the Megasquirt III's datastream and interfacing with Racepak's Smartwire solid state power switching module. Cool stuff.
. The simpler and less expensive G2x is very popular with trackday and club racers.
Other popular systems are Traqmate and their new dash. Max Q data, Race Logic DL1, RLC Commander, Aim Solo and Solo DL. I'm probably missing a few.
If you already have friends with data ac already, give consideration to how useful it would be to able to share and compare data using the same software. That can be more valuable that any cool feature you see on the hardware.
Another reality these days is that you may not need any of it. The latest smartphone apps are truly amazing in how much data they'll capture. The latest apps are just about ready to read your OEM ECU data, collect physics data and record HD video all from your phone. Finish the session and it's already combined with a data overlay showing lateral G's, track map, rpm, lap times. There is at least currently, a limit to the resolution and number of channels any phone can handle so if you want true data ac, you gotta buy the hardware.
Here are some commonly used channels. Decide which are important and how you will use them. That might help narrow down things.
included with most entry level systems
Predictive lap timing (my favorite feature)
Best Lap display
usually requiring extra sensors or adapters for your OEM sensors
EGT - single or multiple
Water/meth injection reservoir level
Advanced physics data
Brake line pressure