1st Edition
An 'Edition 1' stamp sits on the left edge below the artwork. It is the rarest and most sought-after version of the set.
The 1999 Base Set is where the value gap between a raw card and a high-grade slab is the widest in all of TCG. It is also where disappointment is most common: Wizards of the Coast print runs are notorious for loose cutting.
In short
A Pokémon Base Set card (Wizards of the Coast, 1999) is graded on the usual four criteria, but centering is by far the limiting factor: the print sheets of that era were poorly calibrated, and a significant share of copies exceed 65/35 on the front, which caps them at 8 with PSA. Before submitting, identify the version first — 1st Edition (stamp to the left of the artwork), Shadowless (no drop shadow to the right of the frame), or Unlimited — because it completely changes the value, and therefore whether the submission pays for itself.
The four pieces that carry the 1999 edition's value.

Charizard
4/102 · Holo Rare
The set's benchmark card

Blastoise
2/102 · Holo Rare
Often poorly centred

Venusaur
15/102 · Holo Rare
Light background shows marks

Pikachu
58/102 · Common
Cheap raw, heavily graded
In the order they cost you points.
1999 cuts are irregular. Measure the left/right margins of the yellow border: above 65/35 the 9 closes, past 70/30 you are in 7-8 territory, however good the card is otherwise.
The holo layer on the sixteen shiny cards scratches from simple rubbing in a binder. It only shows under raking light — tilt the card before concluding it is clean.
The plain blue back forgives nothing: a slightly whitened corner or edge jumps out. It is often the back, not the front, that turns a 10 into a 9.
Shadowless and Unlimited look alike at a glance. Getting the version wrong means being off by a factor of 3 to 10 on value — and submitting a card that will never cover its fees.
An 'Edition 1' stamp sits on the left edge below the artwork. It is the rarest and most sought-after version of the set.
No stamp, but no drop shadow along the right edge of the artwork frame, and a thinner HP font. An intermediate print run, considerably rarer than Unlimited.
A clear drop shadow to the right of the artwork frame. This is the mass print run: grading only makes sense on iconic cards in excellent condition.
Two photos are enough to know whether your copy sits in 9 or 8 territory.