Jeez. I was ready with my E answer...until you guys threw out all those chords up the neck a bit. So now my answer is E, and all the chords mentioned like Am7, etc.
I see a lot of people have trouble with the Bb- it is funny, but that was one of the first "harder" chord I tackled. I just sat down and worked on it over and over and over, slow then faster. I really like the sound of that chord when it comes around and am glad I put the time in. I just can't get myself to do that for the E. Dunno why.
I had trouble with Em for a long time. Not the chord shape really, but getting to it, and back out, smoothly. I am feeling more confident lately though. I'd echo what someone else said- the more I play, even just playing chords I really know, and working on changes, some picking, scales, and everything else, I am finding new chords, songs, strums, come so much faster and easier now. Turns out the practice works. I also find taht I have started to lighten up on my death grip I had on the neck. That opens up a world of chords!