You will likely get as many answers as there are members here. I think that a good NAV system is largely integrated, has some redundancy (particularly on important instruments, such as depth, GPS, radar, & VHF), and fits your available space and budget.
For me, I centered my navigation around Time Zero Pro with 3 19" monitors. Because I wanted radar integration, I added a Furuno TZT14, radar, and multisensor (to get depth & speed). However, I'm not a big fan of Furuno, so I added a Garmin package as well which included 2 GPSMAP 8616s, radar, multisensor, AIS transponder, 5 4" dedicated displays, 4 cameras, & autopilot. This gives me some degree of integrations and complete redundancy where I think it counts most.
For me, this setup works well. I wish I had more space on the flybridge for more displays.
What is still a challenge for me are the various electronic charts that are available (or not) on my systems. For instance, I've found that Navionics has more detail and accuracy than CMAP, but Navionics won't run on TZ; CMAP does. But the newest CMAP charts won't work on my TZ4 system, unless I upgrade to TZ5. Navionics runs on Garmin, so I get the detail in narrow channels on that system. It's always something.
To recap, I'd start with the functionality you want, pick a piece of gear that you prefer and build the rest of your NAV system around that.
I hope that was helpful to you.
Safe Adventuring,
Peter C. Adams
m/v Blue Redd 5726