"Best and worst" are subjective to being with. The "Best" for an
application may fall in the middle of the range if a user values
durability and future parts-compatability over light weight for
example.

Having said that the lines fall as follows (things on the same line are
positioned equally in shimano's lineup) (I think this is right, doing
it from memory)

Road/Track:
all Dura Ace
all Ultegra, 600
all 105
Tiagra, RX-100, I think the "Flat bar" stuff is compareable at this
level
Sora, RSX, 2200

MTB:
XTR
XT, Saint
LX, Hone (Saint and hone are more freeride, Lx/XT are more xc)
Deore, STX-RC
STX (note that stx-rc and stx used to occupy the positions in the
lineup now occupied singularly by Deore)
Alivio, Exage
Acera
Altus
Tourney, 100GS

Nexus is a whole different category for internal-hub components. If
you were to compare them to the other lines in terms of quality they'd
fall anywhere from the altus/sora range through the lx/105 range.

The C-series is designed for hybrid/comfort bikes.

DX is shimano's BMX designation.

I'd also note that there seems to be less variability in the road
lineup than mountain. Tourney is crap. Sora is actually quite good.