For the past ten years, Phoe and I, and a number of various players, have worked with a formal+house rules edition of AD&D, based on the AD&D Core Rules, namely the Player's Handbook, Dungeon Master's Guide, game settings and "complete" series of class-based guides.
The term "Ultimate" is not used in its definition as the "best" but instead refers to the concept of ultimate as complete, and therefore not subject to having new "player races" or new "rules concepts" thrust upon the players. (Although AD&D has ways to construct by-group rules for each.) New spells, new monsters and new worlds are possible, but at no time do brand new rules come out and destroy or change the rules we are working with, although occasionally spells are changed (nerfed) as they are found to be over powered, or, more rarely, underpowered....if you understand the core rules, you never need to relearn a new system although someone (often me) will occasionally have to rule when there are two published sets of rules that conflict.
The canonical material we use is any official AD&D (and to a lesser extent, Original or Basic D&D) product published before the 2000 switch over to "3rd edition Dungeons and Dragons." This includes both original AD&D and second edition AD&D because they are, in fact, the same game, with only differences in the optional rules (such as psionics) and a few cosmetic differences more important to the DM than to the player (combat charts to THAC0, board inches to yards, weight from gp to lbs, value of coins from 200-20-1 to 100-10-1-0.1 .)
Part of this is disgust at abandoning the game we loved. You can pick up almost any book from 1977 to 2000 and insert it into the existing strata and have it work...3rd edition and parts forward cannot be swapped in this manner...even with "editions" that came only a few years later. You can think of it in similar ways to how operating systems work...you can open a program for an early version of windows in a later version of windows, sometimes with a little bit of fussing, and there is legacy support for many programs. WOTC killed Dungeons and Dragons by killing legacy support...there were two earlier Dungeons and Dragons 'operating systems,' (similar in many ways to MS-DOS and Windows, including being able to use pre-3e Dungeons and Dragons inside AD&D if you knew how to work it) and the WOTC 'operating system' didn't ever run this legacy software.
There was an AD&D emulator, to continue the metaphor, in the d20 system, but one of the things that irritated people like me was the fact that the d20 system allowed the players to use legacy materials without ever needing to reprint them. Some of our friends worked damn hard to write modules and materials, and by sitting on the materials, and providing no legacy support, WOTC has essentially gotten the essence of the intellectual property of their authors without actually having to pay any of those authors for their actual intellectual property.
AD&D then, is an abandoned game, a table top version of abandonware, and if WOTC actually gave two shits about the game instead of just making money they'd either make the books available (in POD, for example) and cut the authors a percentage (which would still be profitable) or they'd release the copyright on all the material that they own (as works for hire) and allow the authors to have name recognition, if not new money. (Actually, releasing the works as copy-lefted, and freeing the AD&D name altogether could also be profitable if they just did reprints of core materials-monsters, spell collections and rules.)
If you have any doubt about the demand for these materials, and the fact that they would be purchased if reprinted, look at the number of uploads and downloads of AD&D materials on any illegal or semi-legal download site. Hundreds, maybe thousands, of people have scanned almost every book in the AD&D cannon and made them available. Note I'm just providing that link as an example, I'm not suggesting that anyone, especially my players, download such materials illegally...even if they don't have their own player's handbooks, which can also be purchased used on websites like amazon.com.
So UAD&D is all of Advanced Dungeons and Dragons- spell lists, psionic powers and monsters compiled from all of the published materials and sorted by database or similar techniques so that a DM can have unique worlds and unique creations where players are involved in the world-building experience. It is a full and complete system, where you can't go off of the game board and need a new book to know what's going on. It's a one-book for each player system- players need a player's handbook, and only a DM need blow extra money on having one of everything else.