This has long been an interest of mine as well...
I have never used Agar Agar in ice-creams, however I've tasted some having agar agar in it... I've found the texture suffered a lot, and the flavor release wasn't really good in a cold application.
Xanthan gum is my go-to thickener in many applications; you can achieve almost custardy consistencies with it... And you can to some extend use it as a stabilizer in ice-cream as well, as it prevents big ice crystals from forming.
Guar Gum is poor man's Locust Bean Gum (in a way). It's has similar (or even better) crystal preventing capabilities like Xanthan gum; and it's also a good thickener. You can also combine it with Xanthan gum to achieve stronger thickening properties.
Now, my all-time favorite, even though you haven't mentioned it...
Locust Bean Gum is the best ice-cream stabilizer on its own... It's really expensive compared to other thickeners, and yet it has the best preventing capability of all. It's also an excellent thickener. In combination with many other thickeners (like... Xanthan gum, Kappa Carrageenan) it can form gels.
I basically find, LBG and Invert Sugar delivering the best ice-cream stabilizing effect and a great aid to the texture of it.