Basically IMO it comes down to these 2 general categories.
1) Generic OBD I and OBD II series scan tools. These will give you GENERIC access to trouble codes and data, OBD health checks, etc. Some will also give you code definitions and some troubleshooting which is very helpful (IE the more the better IMO) Usually these come with a few adapters and the generic 16 pin OBD II adapter. This can be anything from a USB adapter for your laptop, bluetooth widget for your smartphone, etc.
2) Scan tools with vehicle specific or manufacturer specific access to the different onboard controllers via the make, model, and VIN # entry offer advantages in the amount of information available to you for diagnosis and troubleshooting. Any of these will cover all of the generic PIDs with the addition of being able to access the manufacurers PIDS in the ECU, TCU, BCM, SIR, ABS, HVAC, modules. More expensive kits will include additional connectors and software for Europeanvehicles, adapters for older ODB I vehicles, etc. The downside with these is price. All the extra software and adapters give you a lot more capabilites and it all costs more too. The detailed troubleshooting software included with my scan tools has helped me more times than I can count over the years as I work alone most of the time.
One specific reproducable example I can think of right off is a 2001 Ford Ranger with a 4.0 SOHC V-6 engine. Pull off the connector for the cam sensor on the left valve cover and start the engine. The generic scan tool connected to the 16 pin OBD II port does not read a code P0340 for quite a while, perhaps miles. With a scan tool connected using vehicle specific (VIN) entry you will see that code P0340 instantly (the second the engine starts).
Lastly, search out the code setting parameters and the information available in generic mode $06. It helps to brush up on hex a bit, but you can actually use this data to verify repair effectiveness. Again, both methods provide valuable information and in my opinion are both needed. Most issues can be resolved with a good generic OBD scanner, but as you see the more you learn, the more toys you seem to want. Good Luck! My diagnostic kit also contains a Lab Scope, Logic Probe, and DVOM too. These are my ramblings. You are free to form your own.