Flaco is a level above all those you mentioned above. He can certainly make a case for best one pocket player in NYC. I put him at Santos' speed.
Slim is a great undercover player. He's also getting up there in age, as is Bill. They're probably about even. I've heard great things about Mel's game, but I've only ever seen him play once and that was against me. He seemed over the hill, but he could have been having a bad day. I didn't really want to stick around for a long session because that room he played at in Lodi, NJ gave me the creeps and seemed very Italian Mafioso.
As far as Bill goes -- and I have no problem calling it as it is since I'm no longer around the game -- he's a snake. Even by pool subculture standards. He'd steer his mom into a bad game. And he's certainly not a gunslinger like Freddy was or I've heard John H. is. If he's in a game, he's 100% sure he's got a 3-ball advantage. That being said, he's a very knowledgable player and I know that Pat Fleming has had him up in the Accu-Stats booth several times in the past. In particular I remember him commentating on a Cliff-Buddy match from 2000 or 2001. He's had a string of top 20 to 30 DCC finishes under the names Henderson and Hendrickson (his real name). As you can tell, I don't have a lot of respect for him as a person, but I do have respect for his game.