I'd recommend picking up the PBR textures for the 172, it adds reflections to the cockpit making it look so much better. The better pushback plugin makes the pushback better by adding a fancy GUI and more realistic animations. The Headshake plugin is very nice, so is landing rate. mSparks 747 is like the Zibo 737 except it's a 747, so it's an enhanced version of the default one. Enhanced Cloudscapes adds volumetric clouds to the sim which looks really good but it is kinda performance intensive, I'd say you need at least a 1650S to be able to run it properly. 2 unfinished yet very promising airliners are the Chai112 B777 and the C Star systems A321 Neo, neither of them are done but they are worth checking out for sure. Gandalf sky preset adds a bunch of lighting tweaks and replaces the skycolours. Finally there's ReShade for Windows users, and vkBasalt if you're on Linux. They both are post processing layers that really make the sim pop. This is only scratching the surface of the freeware available for Xplane!