Cold/Sonic
Rad/Sonic
Dark/Sonic
If you notice the trend here (and in this thread in general), the -Resistance debuff on Sonic is just too good if you are creating an anti-AV toon (which I also have and enjoy). When you're considering a group-friendly toon that allows your team to effectively take on AV's, you'll want to maximize the debuffs to the AV across the spectrum, so that usually entails -Resistance, -Regen, -Defense, -ToHit, -Damage (Usually in that order). Thats why you're seeing all the Cold/, Dark/, and Rad/ suggestions, as all three cover all or a wide range of these effects.
Underrated is usually Dark/ with the -Resistance from Tar Patch (Especially if you're stacking it with Sonic) and the -500 Regen from Howling Twilight, which I believe is still an Auto-Hit (Money!) along with the -ToHit you can essentially floor an AV's accuracy with by yourself.
I personally run Rad (for now), because I like that its a great utility set that includes an easy group heal, accelerated metabolism (+Dmg, +Recovery, +Speed, +Recharge, +Res Mez...MONEY!) along with the debuff toggles (which I tend to like more now that the patch has hit). If you're designing an anti-AV toon, the Rad/ toggles are fire and forget, allowing you to focus on your attack chain instead of constantly working back in your debuff powers like some of the other sets. Not that this process is THAT big of a deal, but thats simply when preference kicks in. I like to put my toggles on, then DPS the hell out of them.
Like some others have mentioned, even with the patch Rad/ can still be annoying if you have a fast group-killing team, but in the big-picture, if your group is killing large mob packs that easily, even with a boss, you have a good team and can just focus on DPSing the boss, as /Sonic is ALWAYS going to be useful on them.