This is my personal ranking for each piece in standard Tetris. My rankings are based on how each piece contributes to the Tetris ecosystem, not just how useful it is. So read ahead, the rankings may surprise you.
The most boring piece. Just a square, no variations. Good for filling in 2 wide gaps and not much else.
6 and 5: S and Z
The most irritating pieces. Causers of gaps and holes in your board. But without these challenging pieces, the game would be pretty boring, so they get to be a bit higher.
3 and 4: J and L
Less awkward cousins of S and Z-blocks. Handy for filling in 2 and 3 wide gaps, and if you’re desperate, 1 wide gaps. These pieces tend to not cause any trouble, but aren’t too exciting to see in the preview window.
So why isn’t this #1? Because the I-block has only one good use: clearing 4 lines. If you aren’t set up for that, its just brings disappointment. During later stages when your board gets really high, it could potentially be a hindrance as well. That said, clearing 4 lines is still the best thing you can do, ensuring a high rank for the I-block.
The T-block is a team player, a problem solver. Each of its 4 positions are great for filling in troublesome areas, setting up for future blocks. This alone puts it in the top spot for me, but it gets cooler from here. Newer versions of Tetris reward players for clever use of the T-block’s rotation to place it into otherwise unreachable locations. These are called T-spins and reward additional points.