I think you only need one imager, really...
It depends what you want to be prepared for, particularly if you are hunting for ground units.
Here's a summary:
Hi-Res Imager: Day + {clear, overcast, vacuum} - {mimetic, CLPS, voidsig, camouflage, underwater} - in {building, heavy+ woods, jungle, smoke, underground}.
IR Imager: {clear, foggy, overcast, cloudy, rainy, snowy, vacuum} - {cold shutdown, nullsig, voidsig, stealth, 4+ depth water} - in {ultra jungle, CF70+ building, underground}
Look-down radar: {clear, foggy, overcast, cloudy, rainy, snowy, vacuum} - {Infantry, ECM, nullsig, voidsig, stealth, underwater} - in {building, heavy+ woods, jungle, underground}
And comparing them we see nothing dominates.
Hi-Res penetrates Nullsig and stealth armor (unlike IR & Radar) and can see shutdown units (unlike IR) or Infantry (unlike Radar).
IR works at night (unlike Hi-Res), can penetrate {3- depth water, CF69- buildings, heavy+ woods, heavy jungle} unlike Hi-Res & Radar), can see through {Mimetic, CLPS, camouflage, smoke, foggy, cloudy, rainy, snowy} (unlike Hi-res), and can see {Infantry, ECM} unlike Radar.
Radar works at night and can penetrate {mimetic, CLPS, camouflage, smoke, foggy, cloudy, rainy, snowy} (unlike Hi-res), and can see shutdown units unlike IR.
Even pairs don't dominate.
Hi-Res+IR misses cold shutdown camouflaged units (unlike radar).
Hi-Res+radar misses mimetic/camo+infantry and ECM+CLPS (unlike IR) and does not work in heavy- jungle, CF69- buildings, or light woods (unlike IR).
IR+Radar misses nullsig or stealth units unlike Hi-Res.
So, you really need all 3 to gain all benefits. The combination of all 3 gives:
{clear, foggy, overcast, cloudy, rainy, snowy, vacuum} - {infantry+stealth+camouflage/mimetic, nullsig+CLPS, voidsig, stealth+CLPS, 4+depth water} - in {ultra jungle, CF70+ building, underground}.