its pretty hard to choose. but for me, its the 2th block

who th said 2th, it's 2nd also the r piece

If we consider SRS Rotation system to be applied here, the J piece would be a WAY better option than the S or Z piece. You set the J piece onto the place and overhang. then rotate it backwards. it would fit and clear the line! Hope this solves this idea.

J piece, defo. Free two line clear at the bottom!

j