A simple character sheet builder I could use to make bespoke character sheets that are either form-fillable PDFs or hosted so they can be shared with players. That'd be awesome.
My favourite form is “all-in-one-book” no matter how thick and unwieldy it is! Dungeons & Dragons Rules Cyclopedia is a classic example, while Swords & Wizardry Complete Revised and OSRIC are modern examples. Consequently, by buying all three of these you will effectively have full OD&D, D&D, and AD&D games.
Old-School Essentials Classic Fantasy Rules Tome is very easy to use. Combination of clean layout, terse writing, and information concentration make for a great reference book. It is a good example of applying technical writing principles to a RPG book. Alas, it also spawned a horde of copy-cats who just imitate OSE look, without understanding the underlying principles of the technical writing!
What is great writing? To me, great would be evocative, inspirational, not too terse and not too verbose, and clear. I don't think I've encountered any RPG book yet that hits all of those criteria.
Now, none of the above are terse, and do require some effort to parse. But boy, are they inspirational! Whenever I'm in a rut I can grab any of the three, and I'm full of ideas within moments.
Drats, I just packed my RPG books because I'm moving them to another location, so I'll have to go off my memory. Three RPG books that have memorable art are:
HackMaster 4e features a ton of art in the vein of 1e, but more extreme, more tongue-in-cheek, and just more of everything.
AC9 Creature Catalogue features art by Jeff Anderson, Helen Bedfod, Gary Harrod, Tim Sell, Brian Williams, Geoff Wingate, and Pete Young. Not all pieces are great (or good), but there is some indescribable charm to them!
Dungeons & Dragons Basic / Expert have served as foundational rules for my long-running campaign Conquering the Barbarian Altanis—101 sessions over two and half years. Later this year we will be transition to original Dungeons & Dragons, ie. rulebooks published prior to the Advanced and Basic split.
Most recent in Conquering the Barbarian Altanis campaign: Hagar accepts invitation to attend dinner organised by the powerful merchant family Namelin. With some time to spare, the part decided to revisit one of the nearby dungeons they haven't cleared out. They choose trap-heavy dungeon, and get trapped without known way out.
The basic line (B/X, BECMI, Rules Cyclopedia) editions remain ever so popular. In fact, the game I'm running at the moment—Conquering the Barbarian Altanis—is using the lightly house-rule version of B/X edition.
The first advanced edition, especially the core books written by Gary Gygax. Packed with gameable ideas!
The second advanced edition and its wealth of splatbooks. Despite many being hit-or-miss, I still like thumbing through them. Some real gems hidden there.
All of the above are more than enough for a lifetime of gaming!