Transfer cases:
Jeep Dana 300:

This seems to be one of the more sought after tcases for the Jeeps. They have a great stock low ratio. And have low range kits that can offer up 4 to 1 ratios, maybe more now. But there seems to a few people out there that have had problems with these kits when used in extreme wheelin. And a not that this seems to be caused by (according to Tera Mfg) to fualty original gears, or poor instalation. This kit usually runs for about $719 from Tera Mfg. Atlas makes their own version of the Dana 300 that seems to be a great all around solution compared to having to modify your own tcase, and still having the weaker stock case. But it will take a good size chunk out of your wallet.
Other problem with the Dana 300 is that it uses a much longer adaptor then most other tcases. With the exception of the Dana 300 used in the 1980 IH Scout, which has the 'TEXAS' bolt pattern that is the same as Dana 20. This is a great tcase to use if you have a T-18.

Scout Dana 300:

This is a hard to find Tcase. And because MOST people know what it is, costs more.
Problem is that there is one thing that everyone seems to overlook. The drive gear. This is the gear that MUST be on the ouput shaft of your tranny. And this gear is VERY HARD to find. Part number 300-8-11....
Best chance of getting one is call around the Scout dealers. But maybe get lucky and find a junk yard with some Scouts. The gear is a direct swap if it is from a T19 or T15A Scout tranny ONLY if it was bolted to a Dana 300 to start with. The years are from '75 to 80'... But don't quote that, I don't know for certain.
And this little gear goes for $90 to $175, so shop around!!!
Another problem that I have encountered is the speedometer drive.
First, it is mounted on the opposite side from stock (passenger side on Scout). I had to move the cable, and still haven't found a way to protect it without kinking it too much.
Another strong point about the Scout Dana 300 is that is uses the same yokes style as a Dana 20, the U-bolt style instead of the weaker strap version that the Jeep Dana 300 uses.

Dana 300 low range conversion kits:
Tera Mfg makes a low range conversion kit for both Dana 300 tcases, but the Scout version costs more, $819 instead of the normal $719.
And now offer in the kits, all 5 gears. There is still a debate as to the strength of the additional gears. There have been a few individuals that have damaged their gears pretty bad.
This will hopefully be solved soon.

Dana 20:
These would seem to be the most common tcases in older Jeeps. And while the Dana 300 offers a much better low range (2.61 compared to 2.03), I still like the Dana 20. Especailly now that there are people getting around to making a low range kit for them.
And there is the swap of using the 2.42 gears out of a Bronco Dana 20. But some of the flashing on the inside of the tcase will have to be ground out to clear the larger gears. This has been done, and most people say they have had no problems.
The Dana 20 is just about the most common tcase, always parts, and you can get an extra one for almost nothing.
Just have to see if these kit that come out on the market can survive the harshness that rockcrawling can do to them.