Charisma is in no way the weakest stat in D&D, core or otherwise. There are far too many Charisma centered power builds for that to be the case. If it weren't for the hit point boost, I would probably say that Constitution is the weakest. How often do you see that as the core stat for a class ability or feat? Sure, if you're playing certain atypical PC races it sets your supernatural ability DCs, but that's about it. Behind constitution, I would probably have to go with Charisma and then Strength as being the weakest stat. Sure, it's great to have a high Strength and certainly there are many powerful things to do with a high Strength, but most classes don't lose much for having a low Strength score.
Edit: Thinking about it, my argument makes no sense. Far more classes are able to dump Charisma that any other stat. But I think the designers have done a good job of making builds based on pretty much any stat viable choices if done correctly.