EM:
60's
Most EM games pre-1970 are pretty simple as far as their rulesets are concerned. There are some exceptions, but complex EM games that are pre-70's that I love to play...
Williams Lucky Strike. Great shots and a lower PF layout that was seen again in Space Odyssey/Mission.
Gottlieb Cross Town. A true player that has that magic balance of skill and luck.
70's Flip-Flop. Four flags/rollovers. Reset from the mushrooms... and repeat!
A close second is Grand Prix. Those spinners never get boring.
Both games are absolutely addicting.
SS:
Stern Stars in a very simple game that never stops challenging you.
Bally Harlem Globetrotters is another deck that just doesn't get old. Drops and spinners. Rip it up!
DMD:
This is another level of "simplicity" and even the most simple games are miles above the previous vintages of technology.
T2 one of the earliest and best playing DMD games ever made. There are so many copy cats of this layout is a testament to the brilliance of Steve Ritchie's design.
I think Avatar and IM (Pimp77 is spot on) are two of the more recent games that really get the balance of play and challenge and don't really need a deeper ruleset.
All these games are players and with the exception of Stars and Lucky Strike, are pretty easy to find.