Damn for me personally, Optimus I found incredibly fun, and pretty easy to. At least in my opinion. I just hate pushing down the front of the cab at the end of the transformation to alt mode. As you feel like you're scratching paint. I needed instructions for him though.
As for Bee, I didn't need instructions for him. I found him very easy, just a little fragile.
Leader Sentinel Prime though, that has to be one of the worst transformers I've transformed. More so, every time I try him. He just gets more and more stress marks. And now some of the plastic is slightly splitting in his feet. I also had to remove his electronics to make certain parts close in that section. So he was just more frustrating to me.