Does Abomasnow have anything good other than hail in OU which other pokemon can't outclass (easily?). Putting Abomasnow in OU for the sole purpose of hail is something I don't agree with. BL and below have far more partners that can make use of hail. If Abomasnow was moved to OU, all those teams would be lost, since there is no other pokemon with snow warning (unlike other weathers, which have the likes of Vulpix and Hippowdon), which would be a huge disadvantage for anyone in reborn making a lower tier hail team (no, I am not the only one). Pokemon like Tyranitar and Politoed are in OU have something else good in them apart from weather. Politoed can specially hit hard and Tyranitar is a walking special sponge with the power of both special and physical attacks. Abomasnow's abilities (it doesn't have much for it apart from offense, leading to physical/special/mixed attacking) can't stand up in OU. Switching into one for hail purpose in battle in ridiculous, since anything with HP fire, along with Ferrothorn, Skarmory, Kingdra and many others can make 1-turn work of it like a Shadow Tag Chandelure can do for most things.
Abomasnow is fine in BL where it is, where it can tag up with Pokemon such as Kyurem (normal form), Glaceon and so on.
Also, best weather in the game? That depends on the situation. Defense is laughable, since the only thing going for resistance (as far as Ice types go, since they're the worst defensive type) is Walrein and Dewgong (both in low tiers), along with Ice Body. Offense is all it can throw at you in most situations, unlike Sand and Rain, which have bulks like Vaporeon and Tyranitar, and killing machines like Kingdra and Excadrill. Why do you think sun isn't used more often either?