EDIT, also forgot the introduction of XC Expert which entered below Pro with Sport below that, am I right?..
1993 is when XC Expert MD came along, the MD is important btw...
XC Expert MD and XC LTD where in effect the same level, apart from the MD part they shared pretty much the same components, so why they just didn't call it LTD MD other than to phase out the LTD naming I don't know.
Expert was the third in the 'MD' range and LTD was the third in the normal now called 'ATB' size.
Oh and the 1990 catalouge should now be in the archive (with the 1993)
Incidentally there has been mention of LX being bottom of the range, but that would assume that 200gs, 300 LX, 500 LX etc never existed? To my mind the Shimano MTB range began with 200GS and went up from there, making LX firmly "Mid range".
Nah 200/300/400 etc are all learning to ride bikes and ride around town and not for mountains/hillocks.
An 'Mountain Bike' (in my eyes and probably Magazines at the time) would have a Deore or similar other manufacturer components.
Of course this contest does not agree with my eyes
Sachs-Hurret info for anyone
Top two would be
New success ATB
Rival 7000 ATB
So Classic 3500 ATB and below should be ok