What are “The Big Three” RPGs?

[Aplus]

1. D&D zero through three
2. Clones of the above
3. TOON

[francisca]

For me:
AD&D and B/X

<big gap>

Gamma World

<gulf of space and time>

all other RPGs

[Piper]

The big three:

1) OD&D 1974 Boxed Set

2) #1 plus Greyhawk Supplement I

3) #1 and #2 plus pickings from each of the other supplements and "Warriors of Mars" and a bit of Boot Hill (1st ed.) thrown in.

I hope this helps!

[Clangador]

1. D&D zero through three
2. Clones of the above
3. TOON

Wow. I haven't played Toon is a long time.