If I have to personally rank Ike's aerials in terms of usefulness, it would look like this...
F-air > N-air ≥ B-air > U-air > D-air
F-air: Most essential for spacing. It can be used as a follow-up to various attacks, including N-air, D-tilt, and U-throw; kill confirming with it is also possible, made even easier by the latest update, thanks to increased damage output and range. Retreating fast-fall F-air is probably my most used attack for covering my opponents' get-up options from ledges, such as jump, roll, and the ledge get-up attack. Like user Aninymouse, I consider F-air to be Ike's best aerial, even before the version 1.1.4. update. It's amazing how far the move came since 3DS version launch. Over the course of SSB4's lifetime, it has gained speed, an auto-cancel window, altered hitbox placements, minor landing lag reduction, power, and range. If it wasn't for SSBB F-air's absurd range, SSB4 F-air would have overshadowed it in every aspect.
N-air: I had the most difficulty in deciding between N-air and B-air as Ike's next most useful aerial, but, in the end, I gave N-air the slight edge. For me, N-air was my main combo starter as well as a secondary spacing tool to F-air. At zero to low percents I would typically follow it up with Jab, Grab, or D-tilt, while F-air, B-air, or even a footstool would be used at around mid percents. The knockback angle change to this move seems to make it easier to footstool afterward. U-throw to N-air at high percent also seems to be a thing now, thanks to the angle change and damage buff. The move now surpasses its SSBB incarnation damage-wise, 10% instead of 9%.
B-air: Powerful, good range, SH auto-cancellable, and surprisingly fast start-up. Due to this, Ike is one of the many characters whose B-air benefits greatly with RAR. Good for spacing, but even better for combos and KOs. Together with F-air, this is my main aerial used after D-throw and U-throw at low percents. The traditional reverse N-air to B-air combo from SSBB in still intact. B-air can also true combo from D-tilt, even at earlier percents than F-air, though timing can be strict and requires frame-perfect precision from the player to pull it off consistently. I wasn't around during the 3DS days of SSB4, but I do know this move used to be 1% weaker before version 1.0.4.
U-air: The large amount of active frames is the main draw of this move, in my opinion. I don't know if I'm not utilizing this move as much as I should be, but, for me, I used it for mainly niche purposes. Aside from frame trapping air dodges, I was able to gain sufficient mileage from it by using it out of a footstool to D-air lock combo, then following up with Aether. The reduced landing lag by 3 frames and improved base knockback from 1.0.8 and 1.1.0., respectively, did wonders to it, though the loss of 14% damage from SSBB is still lamentable.
D-air: Of all of Ike's aerials that transitioned from SSBB, this one was hit the hardest by nerfs. Not only is it 1% weaker, the hitbox no longer lingers and the meteor hitbox has been moved to the tip of Ragnell. If there was anything positive to say about the attack, it is perhaps Ike's only means of locking his opponents, typically after a footstool.
Although they are ranked differently, they all have their uses. Sorry for the unnecessary wall of text.
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