I have a diamond Poltergust and she is one of my favorite characters to use. She just does ridiculous damage and can crank it those blockbusters over and over again. She works best when paired with Surgeon General though, or against some of the more annoying characters who buff themselves.
Which one is Supersonic? Sounds like Big Band but I couldn't confirm in the forums.
I can vouch for fanghoul though. Bio Exorcist is regarded as the best Squigly, but people forget just how massive Poltergust's damage is. I have no clue about Sonic, but Poltergust is definitely a great choice
Don't have Megasonic (if supersonic is him indeed), but supposedly he's a differently flavoured Epic Sax, probably even slightly worse, since Epic Sax works by just doing your typical big band stuff, and to effectively use Megasonic SA you'd supposedly have to dump his ground combo.
And between Epic Sax and Polter I'd say Polter. Because Squiggly. Also because MAXIMUM DAMAGE. I oneshot a 23K FS diamond Untouchable yesterday with a pretty standard gold Polter. Just max your ATK.
Also Squiggly, especially Poltergust, has options - you can build into long combos or not, you can has removals, command throw and launcher, armor break, meter control. Big Band is a one trick pony - he can only do 9000hit combos and stun if he's lucky. If you meet Brain Freeze you cry, if you meet painwheel with grudge you cry, if you meet SG support you cry. The only good thing he has going for him as a makeshift "pls no bb3 me" is the dash invul.
For me personally Poltergust is a better choice at higher streaks.