I have a 1989, 1990 and 1991.5 Turbo.
Which is the best one and why?

 

Is one of them a WH1C? If so it has a wastegate and spools much quicker.

 

I modded a WH1C with a 14cm2 waste gated turbine housing and a 60mm compressor wheel and housing from Gillette Diesel Service. Can't tell you about performance b/c my rig is not on the road. Popular combination though. I like the WH1C because it uses a V band to attach the compressor housing to the cartridge (center section) instead of the giant C clip that the HX35 has. Just easier to deal with.

 

All 3 that I have are H1C Turbos.
What can be done with them?

 

1991.5 will be the biggest compressor but may have a 21.5 cm exhaust housing. Swap exhaust housings from one of the other turbos (should be 18 cm) and you will have the best turbo out of the three.

 

I am running a HX35 with a 60mm Gillett compressor upgrade and the stock 12cm turbine housing. It dosen't like to tow much in the mountains but it sure is fun.
If I were towing more I would go with a 14cm turbine housing.

Gillett sells H1c upgrades also.
Dodge Diesel Parts | GillettDiesel.com

Dodge Diesel Turbocharger Parts | GillettDiesel.com

 

The non-IC turbos ('baby' H1C) have the small compressor inlet.

As Redneck suggested... combine the large inlet compressor (intercooled) with the 18cm ex housing. Then you've built yourself a stock '93 turbo.

Be careful what you spend on an H1C. Every used WH1C or HX35 (12cm ex) I've picked up has cost less than a new exhaust housing by itself.