BBS is the hardest game in the series on Critical.
You obviously don't know what you're talking about at all.
Haha what? Critical on BBS is a cakewalk compared to the Final Mixes of 1 and 2. When I played BBS I literally had to use the No EXP ability to keep the challenge satisfying, even on Critical.
As for which games you have to play: All of them except the DS ones are important. Basically meaning KH1, KH2, CoM and BBS. Also, either play KH2 Final Mix or watch the 20-something new cutscenes on Youtube.
If you liked all of the other games you will also probably like the DS games mind you. They're side stories but that doesn't mean they don't pack a punch plotwise.