I've only been playing NU for a little while, but I am currently running a Sunny Day team and I must say that Liepard, Volbeat, and Illumise ( I suppose) should move up to B rank. I can saftely say that there are no better weather inducers in NU. I've found Regirock to be surprising poor (It can't put up Stealth Rock AND set up Sunny day, it's too much for it.) In such an offensive team style as Sunny day, it is vital to get up Sun ASAP obviously and these pranksters are the best options for doing so. I assume that when people think of Volbeat they think of Tail Glow passing (which I'd admit it probably belongs in C rank for that role) and Liepard in regard to those silly swagger / thunder wave / foul play sets, but they are unrivaled when it comes to weather set up.
Volbeat / Illumise and Liepard are equal in my eyes in that they both have qualities that outshine another. First of all, they both have encore and access to Sunny day and Rain Dance. Both have Thunder Wave and U-turn. This alone makes them incredibly similar.
Liepard's advantages include access to taunt, which isn't that useful when you have encore, but it does help against a few specific pokemon like hazard setters. It also has a different typing which allow it to better handle certain threats. It especially comes in handy when facing Haunter, Musharna, and Driftblim which otherwise give Sunny day teams some trouble. Liepard is pretty much a full stop to both of them. This is important because Driftblim can lay the hurt on Sunny day teams if played correctly. It is also neutral to stealth rock which helps but liepard probably won't be coming in that often because 1 or 2 rounds of Sun/Rain is really all the team needs.
Now what Volbeat and Illumise have over Lieprad include Roost, which is huge because it allows it to encore a weak attack or status move / hazard and then heal. Stealth Rock is bad for it, but it can heal off the damage. They can also afford to invest in defenses unlike Liepard, who really needs speed investment, and makes them surprisingly resilient. Volbeat and Illumise are more durable than Liepard alone, yet they complement each other well.
Overall, Liepard is suited to typical Sunny day teams better because it emlinmates those threats I outlined which is huge. Also, the best sun sweepers have a weakness to a resistance or immunity to Liepard, which means it can easily encore and set up Sunny day. For exmaple, Liepard can encore Shadow Balls / Sucker Punches / Shadow Sneaks etc. used against Exeggutor, and Psychics used against Victreebel, making it rather easy to keep up Sun.
Although I can't speak much for Rain, Illumise/Volbeat probably fare better in Rain Dance teams because they survive Fire moves better and have Bug STAB for the Grass types like Ludicolo, but I use Volbeat on Sunny day and works just fine. They require almost no support whatsoever, you just obviously need things to abuse the weather. I think they both belong in B rank.