For new players (like, people who hasn't beat en 6-x or find 6-x really really hard)

I think the best practice maps for mechanics are as follows:
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2xp0 - She said
-An easy way to practice trapezoids and triangles (easy)

Sta - Hall of Mirrors
-Teaches how to play relying on ears rather than sight (Except for THAT part, it's fairly doable, Easy)

Waterflame - Jumper
-One of the best way to learn Trapezoids (fast but deceptively easy)

Pikasonic - Emptiness
-Best way to learn how to do turns (90 degrees. Intermediate)

Xomu - Lanterns
Strong general purpose training map (intermediate)

Cube - Star
-There's two, and both of them are just a very good general purpose training map (intermediate, Hard)

Plum - As time Goes by
-Learn to read corners fast (advanced)

Ampyx - Journey
-A mostly slow bpm map that has speed ups. For those who wants to start challenge themselves and learn how to play dynamic rhythm.

HG Cat - Linearity
-You probably can't do this, but this is the ultimate stamina/consistency training map. Go into Editor and set "Pitch" from the menu on the left and set it to 30. 100 is normal speed etc. Increase or decrease this pitch as necessary until you find a speed you can manage and start practicing. (Unreasonably hard)