Simplicity as a criteria?
Let's see. Short rules, simple and self-consistent mechanics...
Fate Accelerated
Fate Core
Starships & Spacemen 1E (not 2e)
Tunnels & Trolls
Feng Shui2
Barbarians of Lemuria
Og Unearthed
Teenagers from Outer Space
Mongoose Traveller - the explorer edition. (which is the essentials and only 2 careers - if you like that, expand to the main core rulebook)
Pendragon Book of Knights (not Book of Knights and Ladies - that's a supplement)
All of the above are findable on Drive Thru RPG (drivethrurpg.com)
The current editions of T&T, Fate Core, and Feng Shui don't look short, but the mechanics are a fraction. of the book.
In the case of Feng Shui 2, most of the core mechanics are in the free RPG day release Task Force Double Eight. 24 pp of learn to play adventure. And a gentle introduction to the setting... It's no longer free ($5), but it's enough to know if one will like the full up $20 rulebook... about 50 of the 350pp rulebook are actual rules; 72 alone are th3 36 archetypes presented as two-page sheets. (If you go for the expensive physical, get the archetype pack to be able to print them easily...)
Fate Core and Fate Accelerated have excessive white space for my tastes, but are very good at teaching the mechanics.
S&S 1E is 80pp of old school goodness... it's a "didn't get the license" simple but evocative Trek game.
TFOS is, IIRC, about 90pp. But it's a total hoot with the right players, and it was awesome when I was in HS... except for the "Ok, how much cash is in your pocket" part of character gen.
Deluxe T&T is a huge book....360some pages... but the rules are actually quite simple. For the first time, it's an edition with a lot of (very optional) setting material. It's easy to play, and DT&T is the best written version. And all the T&T adventures from all editions are easily used.
Pendragon Book of Knights is an inexpensive slightly simplified Pendragon 4E alternate core... about 30 pages. Very little setting material. But it's a great set of rules for playing knights. If you like it, it easily can be used with any of the adventures from any edition, and while it's Pendragon 4 rules, the differences between editions are almost entirely about character generation and pricelists. So, it's a useful starter for any edition.
Material needs for the above:
d6 only: T&T, Og, Feng Shui 2, BoL,, TFOS
d6 & d20: Pendragon, S&S,
4df: Fate Core, Fate Accelerated
Pendragon also benefits from a good map; the archive of Greg Stafford's Pendragon Site
http://gspendragon.com/Pendragon-1st-Ed-Map-Remastered.pdf
Note: you don't need to have them printed out if you can display them as needed, say, chromecast them, or put them on a tablet, to get the utility from them.
www.nothankyouevil.com
