What is the hardest, non-hydrophilic filler?

Your question is very perceptive! Essentially, there is a trade of between the G' value and the hydrophilic nature of the gel. This means that the 'harder' gels are more hydrophilic. Foe example, Juvederm 4 Ultra would be quire 'hard', but it is very hydrophilic and should never be used in the tear trough area. The key thing for the tear trough is a low hydrophilic profile, which means a low G', unfortunately. You should ideally be using Belotero or Teosyal Redensity II for these areas. you need to accept that if you want to eliminate swelling, the filler will be softer. If done well this should not be an issue and you should still get a great result.
There is generally a very poor understanding of the physics of fillers among practitioners, but I hope this explains things for you.

These fillers (belotero, volbella, restylane silk) are good for under the eye, but keep in mind, a combination approach will give the most improvement. Fillers, lasers, and microneedling/PRP can improve under the eyes. My patients try to get non surgical treatments to avoid surgery. We use PRP and stem cells there often. I suggest consulting with an expert to develop a treatment plan best suited for your short and long term goals. Best, Dr. Emer

Dear matthowes, bags under the eyes is the number one complaint of people as we age.
To treat the area, it requires an experienced clinician and multimodal therapy based on severity. Sometimes surgery is the answer, other times a light filler with a peel or laser will do the trick.
Always make sure you know the credentials of your clinician and what products they are using. In most cases I use Radiesse.Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery