The Simrad AP70 seems to get good reviews, but it's a pretty high end pilot and $$. The AP24/28 was a long time workhorse, but is now end of life and replaced by their NAC line. The NAC line is unproven, so I personally wouldn't want to use one until many had shaken it out. Other than the AP70, the Simrad pilots interface with other equipment via N2K only, so if you have 0183 on your boat it's probably not the best choice.
Furuno makes an excellent pilot in the NavPilot 700 series. It's my current favorite. It has very flexible interfacing to other nav systems, so no vendor lock in.
Garmin makes a pilot, but I think it has some number of proprietary interfaces between components - more so than other pilots. So I'd check that carefully before attempting to use it with a non Garmin system.
I haven't used a Raymarine pilot in years, but the one I had was good. I think ray is N2K only as well, but not 100% sure. Maybe someone else knows?
Comnav makes pilots that many people swear by. I haven't had the greatest support experience with Comnav, so they wouldn't be my first choice, but like I said, many people are very happy with them. Comnav is 0183 interfacing only.
I think those are the major players.
The major components of an AP are:
- AP Computer. This box is the brains of the system
- Control panels. Most systems support multiple control panels so you can place them at multiple helps (pilot house plus flybridge). If your AP and Pilot are current model and the same brand, you can usually use the chart plotter as an additional control panel. But these MFD control panels are often not full function, so a dedicated panel is still needed.
- Rudder sensor. I think all inboard APs prefer to have a rudder indicator. Some support "virtual feedback", but performance will be much better with a rudder indicator.
- Heading sensor. This is a critical part of any AP, so don't try to cut corners and cheap out on the heading sensor.
- Steering pump. This assumes you have hydraulic steering. You need a steering pump, and it needs to be sized to match the displacement of the steering rams in the boat. That's so it can move the rudder fast enough to effectively steer the boat. The pump gets teed into the existing steering hydraulic lines; left, right, and the return/makeup line. When the AP is active, it steers by controlling the pump. You helm wheels don't move, and it doesn't matter how many helms or rudders you have.
If you have cable or chain steering, there are other drive mechanisms available, but I'm not familiar with them.
All APs can operate independent of your chart plotter, but they can also be interconnected. If interconnected, there is an AP operating mode where it will follow the waypoints that your plotter dishes out as you follow a route.
I think those are the major points, but I probably missed something.
