This question is something the forum intends on narrowing down in the coming years. Some brands of Hyaluronic Acid (HA) may not be accessible to all Clinics (i.e. regional restrictions/medical bureaucracies), and others may just be a Doctor's preferred choice due to getting a good deal on supply.
That said, in my talks with my Sponsors, they are keen on getting the most "penis-specific" (my wording, not theirs) HA, even if it comes at a slightly higher cost. What I mean by "penis-specific" is a formulation (cross-linking) that seems to mimic the natural penis the most.
As for the "squishy" concerns, I think those are a bit overblown and are usually a result of over-filling and/or using a poor iteration of HA. While we may not know the "best HA for girth enhancement," I can tell you one to avoid: Macrolane. This HA brand has been the common denominator in most HA complaints I've seen in the past.
Also worth taking into consideration is that the natural penis is never stiff like a rock. Even while erect there is some "give". This is why a rubber dildo more adequately simulates a penis than a hard plastic vibrator. Furthermore, most accounts I've seen that are female (or likely authentic female) suggest that the "feeling of being filled up" is what they notice most from girth, and for some women, there isn't a whole lot of inner-vaginal wall sensation (and why so many cannot orgasm on penetration alone). This further weakens any concern I'd have with the texture or firmness with HA. You're looking to stuff her, she won't notice if your penis skin is "slightly off" than the last penis skin she touched. Women aren't nearly as critical of our penises as we'd like to believe they are... we're our own harshest critics. And if you're a guy looking for other guys, I just think there is too much variety and diversity in penis shapes & sizes to warrant scrutiny unless (a) you invite it by acting like you have something to hide, or (b) have a bad result. If you own your newly enhanced penis, you'll only get compliments, trust me.
By all accounts, (short of someone having a very squishy or very irregular-looking penis) the added girth from HA both provides the girth the man wants without the concerns of long-term risk.
As I stated in my opening sentence, no one knows for sure which HA is the best HA. In the
Medical Expert Opinions
page, there is a question re: favorite brands, and I'll see if I can get more Sponsors to update that query.