Which is your preferred version of Killer Croc?

[Restingvoice]

I only remember Waller said reversing the effect of Croc's mutation isn't actually hard but I don't remember why she didn't do it. Probably Squad purposes. Croc was under Waller's watch at that time.

[JBatmanFan05]

and the gangster niche is already full with Penguin, Two-Face, Black Mask and Carmine Falcone.

Croc existed (and as a mob boss) before Black Mask and Falcone were even created. And he was a "mob/gangster boss" (depending on how you define that) before Penguin or Two-Face were (at least in the Bronze Age and Modern Age). It wasn't until the 90s really that modern Penguin became a consistent mob boss (basically the same deal as far as Black Mask). Two-Face has been in and out of that mob boss roll, depending on the writer, but mostly after No Man's Land.

[ironman2978]

I always liked The Batman's version: a mobster with the muscle to make ridiculous plans happen.

Same, I like either the intelligent mobster who plays on his looks to his advantages (The Batman, Beware the Batman) as well as the sympathetic elements of Killer Croc, being Roy’s sponsor and protector of the homeless. Even The Batman of Arkham and Batman: Earth One volume 2 had great takes on Croc, just a guy who wanted to be left alone but because of how he is perceived, he is forced to hide underground. I think the only times I like the mindless cannibal take of Croc was the first Arkham Asylum game with that Peter Pan “tick tock” reference whenever he came close to you. Otherwise, I prefer the deformed man over the giant alligator looking beast

[ironman2978]

Has any comic attempted to bridge the gap between the Pre Crisis gangster and the Post Crisis sewer dwelling monster?

I guess the closest I remember was the New 52 Villains month Killer Croc story: "Batman And Robin" #23.4. There has been a few post crisis that tried to balance the two but none to the level of that Tim Seeley’s comic while adding the element of sympathy.

[K7P5V]

Yeah, but to their credit, the Bat-team of the 90s did mostly okay by the character. It wasn't really until Hush that all this started.

Guess I'm in the minority, but I really enjoyed how Jim Lee redesigned him...

And not just characterization, there's something about Killer Croc's original design that is just so classically comic book and identifiable, paired with the whole gangster motif. Making him generic lizard monster was lame.

In agreement with you about Killer Croc's original design.
When reading Conway's version, I remember finding him rather repellent & unnerving (but strangely irresistible):

[GamerSlyRatchet]

Beware the Batman probably combined the best aspects of Killer Croc, at least to me. Still a mobster, though one that has made his empire within Blackgate Prison instead of Gotham. He's large and animalistic, but nowhere as much as the comics, the Arkham games, or even The Batman. He's well-spoken and mostly chill, but can also go berserk if he needs to. Also probably had the best accent.

[Robotman]

I prefer Croc to have an actual mutation over just having a skin condition. It better explains his ability to swim fast and his increased strength. If he’s just a huge guy with a skin condition he definitely wouldn’t be able to swim very well. Not a fan of the mindless cannibal Croc. I think he should have animalistic tendencies but is still smart enough to lead a crew (or help Roy get sober!).

[Shadowcat]

Conway’s gangster version. Monster cannibal is just plain nasty.

[Baggie_Saiyan]

The Batman Animated Series for sure, in fact a lot of the villains redesigns for that show were so awesome! (Such an underrated show especially the first 2 seasons)

[MajorHoy]

I'll also go with the original pre-CoIE gangster version.

By the way, here's his entry from Who's Who: The Definitive Directory of the DC Universe Vol. V (July 1985):