In my opinion the best monitor aspect ratio for display Amiga modes is 5:4 (the same as native). Unfortunately this ratio is only found in 17" or 19" monitors. I use XTREME mode (made especially for Indi AGA) for Workbench (1280 x 1024) on my Dell 1708FP with the same native resolution. It gives me just perfect, razor sharp display.
For games and demos I have defined 640 x 512 VGA mode. Then Indivision upscales the image 2x and monitor again upscales 2x, adding a bit of blur, which is nice effect. Image takes the whole screen without any borders.
Of course gaming experience is better on big displays, but then they are mostly 16:10 or 16:9 so one either has to stretch the image horizontally (which is done by monitor) or have vertical black bars on the sides (which can be either done by monitor or Indivision).
