I suggest an A seat in row 4 or higher. 1A has no under-seat storage (no seat in front) as well as no adjacent overhead storage, 2A has no adjacent overhead storage (next to galley), and 3A competes with bulkhead 3BC for limited overhead storage nearby. As others have noted, 1A and exit row 12A have the most legroom, if that's important to you. The FA will often store your item in the closet if you're in 1A/2A and polite.

The under-seat storage space is rather small, so if you expect a backpack to fit underneath the seat in front of you, it needs to be a small backpack. The valet check at the airplane entrance works very well.

My home airport (Champaign, IL) is served only by ERJs, so I'm on them a lot. Personally I've never been moved for weight distribution purposes, or if I have, it was so long ago that I've forgotten.