Hmmm...I agree with the "gentle giants are made not born" statement by Solon.
I got my first horse, a Percheron mare, in June 2009. Now, I have heard that Percherons are a very smart breed, and have more of a reputation for mischief than the other drafts breed, and I would vouch for that completely. She is amazing with children and dogs and will take care of a beginner wonderfully. However, with me, she is a different horse. Spooky days, energetic days...she can be as goofy as any light horse. And with my mom, she'll basically drag her over to food, because she knows my mom isn't an assertive horse handler and won't do anything about it.
She is by no means a "mean" horse, but she is however very intelligent and knows it. She is a very fun horse, though! I'm not sure how many beginners could have been successful with her - I spent tons of time researching and asking others for help with her, and I honestly don't think I had enough experience to deal with her. It could have turned out very poorly without all the external help I got (I don't mean training, but I asked a lot of questions about what I should do in certain situations. I still did probably 70% of the work myself).