It would come down to personal preference. No one game is the best looking ever simply because of subjective opinions on art direction and what the game is doing. In that way there's no real wrong opinion.
Flight Sim definitely seems to be the best looking game on Xbox though and one of the best looking console game out there for what it is doing. It's a flight sim of the world so on a macro level, it looks amazing because of how far you are viewing the assets. When you fly near the buildings and grounds though, the GFX are almost OG Xbox / PS2 level and that's because you are not meant to see them that close. The grand scale of what Flight Sim is doing is technically impressive too. I wonder if the enhanced graphics package you can buy would fix that or is that meant for something else?
In a way, you can't really compare Flight Sim and Rift Apart because they are two entirely different genres of games. For me though, if you really want to compare the two, Ratchet and Clank Rift Apart bests it because of how the game runs and looks while supporting raytracing + real time reflections, 60 fps, LOD0 (same assets in gameplay as in cutscenes), high poly textures, unprecedented visual density packed in one screen, particle effects, animations + NPC/AI calculations.
This doesn't take away from what Flight Sim is doing or how it great it looks but to me, the amount of stuff going on in the screen while playing Rift Apart makes it visually more impressive in terms of sheer graphical/visual fidelity.