Started typing and this long-*** post is what happened. Sorry...
A lot of good info here. It does vary a lot of vehicle, tire size, terrian, engine torque specifically, and transmission type and gearing.
With mine, I ran a larger tire (35") but felt I was just perfect at 63:1 with my diesel! (This would be considered quite low, for example, by Scott's standards). But that is here in Utah, lots of dry terrain but I ran everything, mud, you name it, and the ratio challenged me to enjoy wheeling realy hard stuff but also did very well day to day and all terrain. This is what you should be after. No compromising gear ratios.
The beauty of the standard is the diverse range of gearing. You can be fast and slow with a shift... (I'm sure this is fine too with an auto but you always start in the lowest gear). My setup was: 5.61 first (granny) with NV4500. 2.72 Low Range with a NP241 transfer case. 4.1 factory Toyota diffs = 63:1. I am going with almost identical gearing again this time, but I have a choice of going lower with a 4:1 kit. I agree this is probably too low in reverse in particular, and it is cheaper to stay stock.
With the diesel at super low RPMs... My off road style since I've owned diesels, is idling over everything. Left foot brake in gear to slow down to even 400rpms (it chugs over anything, it is like a wall of unstallable torque), then if I need fuel I bump it or throttle it accordingly. It can go from low RPMs with tons of torque to upto the 2500 rpms governed limit. It is like a V8 at 1000-1300 RPMs going to 3500 rpms. It is a "common complaint" with American rock crawlers that diesels will not have high RPMs for throttle up things. The diesel is a little less top end than a V8 but works well and is comparable to a gas engine it you run it this way. I never found it to be an issue. Also this low chugging is easy on parts, very little breakage. But crawling in this style only works in high traction situations like dry high traction terrain and all over hte Southwest for example.
But this ratio for me was literally perfect and was universally great. There is only one place at one time that I particularly wish my ratio was lower, on dump-n-bump on the Pritchett Canyon trail in Moab. I had to brake my way as slow as possible (but still nailed it fine). And also if I get into some really really hairy large boulder wheeling like Montrose or something similar, which has happened a few times... My only complaint about the whole combo was a wide spread between 3rd and 4th gear (and generally awkward gear spreads of the overly-wide NV4500) so I solved this by purchasing a NV5600 this time which adds an extra gear between the 5.63 low and .73 overdrive.
I have played and messed around a lot with gearing ratios. I also drove stockish trucks for years with manuals and very fast low ranges, 28:1 and 30:1 and such. A gain, IMO learned how to drive, how to get up and over stuff, and for years with stock rigs I did fine. You learn how to slip the clutch, bump over things etc. It is not the prettiest method but you learn. The biggest issue with these is the lack of a decent crawl ratio. Trucks designed for off roading in Mud and needing speed, rather than grace... The only appropriately geared truck I've even owned out of the package was a Series III Rover (never owned an automatic).
My last built Cruiser (the sprung over FJ55 on 35's) I was very comfortable and easily hung in there with way more built rigs and always got over most things with this 63:1. I was also frequently told that my combo felt like it was 100:1 or deeper on a built rig with large tires, so it was the best of both worlds. You could get out out and walk along side it as it idled up a hill.
I also did some massive mud trips (one night the truck spent the night in a bog near town, 2 feet deep in a bottomless pit, the other two trucks were stuck too, nothing to winch to (why I now own a Pull Pal!). And Kurt and Dave Connors and I did a La Sal trip out of Moab one night with me and Kurt with our 35's taking turns breaking trail up maybe 3' deep snow way up there... The diesel let out a lot of smoke at higher RPMs with the wheels spinning away and high RPMs only (no traction) but did great. It was an awesome setup..
For a gasser, the issue is torque (or lack of it in certain applications). This is why manuals are less comfortable with gas engines, because they stall easily (stalling = lack of torque). As they say (Ferrari), horsepower sells cars, torque wins races. Some engines have a lot less torque than others and stall out easily. If this is the case you want a high gear ratio, this increases the torque (torque x gear ratio = actual torque at wheel) and lets you get over more. This, the gas engines they run, and the massive tire sizes the guys on Pirate4x4.com run, and high traction, means most of them want deep t case ranges.
Automatic gearing is less important but is important too, in my opinion. I have heard excellent comments about different first gear ratios in autos and it does matter. The beauty of autos is the torque converter and slipping. I consider automatic gearing to be more of an issue of "balance" rather than "capability." So basically any gear ratio works to an extent, but different t-case and axle combos combined with different tire sizes makes them happier and perform stock-like. My philosophy, at least with Toyotas with autos but probably any other truck too, is to modify gear ratios accordingly to tire size. So with FZJ80 Land Cruisers, 35's with 4.88 axles, no other changes. Stock tires with stock 4.10s. There is a big difference between FZJ80s with 35's and stock 4.1 axles and 4.88. The 4.88 is much happier with 35's. So the crawl ratio goes "deeper" so it might seem more competent, but is really just trying to get back to stock or just a hair below it.
I hope this all helps and is what I have learned over the years...