It was hard to pare it down to only 5 so I have 2 honorable mentions. I also think it is important to remember this is 'iconic' covers not best covers, so I took into account how the covers played out on the whole comic landscape.
So here we go:
Close but so far from the official list: Supergirl #50 (1996, PAD version); Adventure Comics #381, Final Crisis #3
Honorable Mention #2: Supergirl and the Legion of Super-Heroes #23 (Adam Hughes variant cover)
There is something majestic about this cover by Adam Hughes who downplays his cheesecake factor here. Smiling, happy, basking in the sun while riding a meteor ... this image invokes all the right feelings about Supergirl.
I know Supergirl's role in that Legion (heck that iteration of the Legion itself) was brief and may be forgotten over time, but this cover simply works.
It ended up on a t-shirt and made into a poster.
Honorable mention #1: Action Comics #252
I don't know how her first appearance didn't break the top 5. The comic itself is a hard to find collector's item and an obviously important piece of Superman's lore.The image of Supergirl flying from the rocket as she lands on Earth has been replayed on other comics, Supergirl #75 (1996 series), Many Happy Returns TPB cover, and Supergirl #1 (3rd printing, 2005 series).
This image has also been immortalized as a DC Direct statue and even a Supergirl bobble-head doll.
Number 5: Superman Adventures #21
There was a time when the DCU was being actively promoted by and influenced by the DCAU and Bruce Timm was the architect of those changes.
When he introduced Kara In-Ze to the DCAU, it introduced Supergirl to a sector of fans who may never have known that Supergirl existed. Garbed in a white half shirt and red combat boots, this version of Supergirl became a big part of both the Superman animated series and the JLU series as well.
Her costume became so synonymous with the character that Peter David had his Linda Danvers don the white shirt to streamline the look of the character. Gone was my beloved Matrix version of the uniform.
This cover, done by Timm, was the first appearance of that Supergirl in comics. It has since become a t-shirt and poster. The animated Supergirl has also been seen in numerous toys and statues.
Number 4: Supergirl #1 (2005)
I had to include on Michael Turner image since his is the current look for Supergirl. Plus, it had been 20 years since the original Supergirl left the DCU in Crisis on Infinite Earths and so for many fans this is *THE* Supergirl.
This version of Supergirl #1 is such a striking cover that this is the image that pops into my head when someone says 'Michael Turner Supergirl'. The shading and blurring out out of the colors at the extreme margins of the picture gives it an ethereal feel.
This image also made it onto a t-shirt.
Number 3: Action Comics #285
I had to pick a Silver Age image to represent and Action Comics #285, the issue where Supergirl is revealed to the world, is one of the more famous covers. Personally, it is my favorite.
Supergirl had matured over the 2 plus years after she was introduced and basically solo'd in this comic.
This issue also has cameos from John and Jackie Kennedy as well as Nikita Kruschev.
This cover made it as the cover image for Supergirl Archives Volume 2, Showcase Presents: Supergirl Volume 2, and onto a t-shirt.
Number 2: The Daring New Adventures of Supergirl #1
This happy cover by Rich Buckler is another one of the key images in my mind of the character.
When people say 'pre-Crisis' Supergirl, this is the image that comes to mind. The 'hot pants' costume is the Bronze Age costume in my mind, not the headband look.
When you reread Daring New Adventures, Supergirl is very confident and strong and acts as the sole major superhero in the Chicago area. Those are good memories for me.
This cover was immortalized as a U.S. postage stamp, perhaps the most widely visible representation of Supergirl in the mainstream public.
I really love that cover and was glad to get a Rich Buckler commission for my collection.
Number 1: Crisis on Infinite Earths #7
While I know this cover represents the death of the Earth-1 Supergirl, there is no denying that this is the most famous Supergirl cover, instantly recognized by the most casual fan as Crisis #7. Everyone knows it.
At the time, Supergirl's death represented the shocking new reality that no one was safe in COIE. Sure all of Earth-3 died, as did some Western stars, and some other B-listers. But this was the big time, a huge character in the DCU falling. Worlds will live ... worlds will die, indeed.
This cover was made into a DC Direct statue.
But more importantly, it has been emulated many times on comic covers, a mark of how striking it is. We have seen versions of this in many places Supergirl #79 (1996 series), Superman: The Man of Steel #10, Firestorm #21 (2004 series), Valor #18, Fallen Angel #16(volume 2) and even Major Bummer#12. With all those homages, it clearly is the most iconic Supergirl cover.
So what does everyone else think. Any other cover that should cracked the top 5?