I guess it sorta depends on what you have planned for this bike.
The top of the line touring/ATB derailleur for 1985 would have been the XC. Just plain ol' "XC." It's going to have the highest ratio of durability to weight ratio. It also came in a triple pulley version- it can wrap STUPID amounts of chain. And it shifts supremely well.
The Cyclone MII is going to be a more gracile unit- I had one on my Trek 720 for a few weeks- It worked absolutely fine, my thought was to be doing loaded touring- and it just didn't seem as burly as some of the other choices.
The reason I choose a Suntour derailleur after 1984- in the early 80s, Suntour was messing around with triple pivots- trying to mimic the shifting of the Huret Duopar. You'll notice that derailleurs like the Mountech get a bad rep because they had too many moving parts and were too complicated for things they didn't need to be complicated about. So, as cool as a LeTech is- I don't like that great big long cage connected to a sprung pivot connected to another sprung pivot that then is connected to the sprung derailleur body.
So if you're planning on using the bike as a light tourer/around town bike- go with the Cyclone MII GT. If you're planning on doing a 'loaded touring/ruff stuff' type thing- go with XC.
If you're not being tied to particularly period correct and/or not Suntour.... The Suntour XC Pro and XC Comp from the late 80s/early 90s are the king **** of ATB derailleurs. IMO they're classic looking enough to look at home on a mid 80s bike, and they're just plain beautiful units. The M700/M735 Deore XT derailleur is pretty much the standard to which ATB/touring derailleurs have been measured. The M900 XTR is an upscale version of that- but only comes in a medium cage (I keep saying I'm going to try a cage swap, but haven't gotten around to it.). The Deore MT60 and 62 as well as the Deore DX M650 are really nice units just under XT.
Again- IMO- after 1994 stuff starts looking a little to "modern-y" on mid 80s bikes. It'll probably work better- but there's an aesthetic trade off. Again, just my opinion.
Suntour XC triple pulley:
Suntour XC by
Dave The Golden Boy, on Flickr
XC Pro:
Suntour XC Pro Medium and Long Cage by
Dave The Golden Boy, on Flickr
A medium cage Cyclone MII, XC and LeTech
Suntour Derailleurs by
Dave The Golden Boy, on Flickr
M735 XT with M900 XTR
XTR.XT by
Dave The Golden Boy, on Flickr
Was that a dorky enough answer?
