Tubis

Tubis is a complex Tetris variant that is best described as the "expert" level of Welltris. It is best described as Welltris stretched out in every direction so the well becomes cylindrical (a tube, hence the name Tubis) instead of cubical.

Perhaps the best way to describe why I think this is THE most complicated Tetris clone of all time is to describe how the game works. The playing area, i.e. the tube, consists of 16 rings, each comprising 48 gray block. The rings are divided into six arches (of 8 blocks each) and each arch has its own color which is defined by the color of the corresponding arch on a colored ring of the tube wall. Blocks of various shapes and color, as in Tetris slides down from the center of the tube towards the outer wall. Your job is to rotate them around the tube to "clear" the blocks by completing a ring.

What makes the game more difficult than it already is, is the requirement that you must make a ring made out of blocks of the same color to make it disappear. On top of this, the speed of falling blocks increases exponentially with each new level. If you're a Tetris expert who prides yourself on having beaten every variant in existence, Tubis is a perfect challenge. For the rest of us, though, the game is simply too difficult to be fun for more than a few minutes.