-500 HP at level 1 for a ranged champion
-55 Base AD at level 1 (same as Garen and Xin for some reason)
Base stats are nearly identical between champions. You have to look to the extreemes to really find much of a difference early.
For mental math, it's safe to assume a champion has:
~500 + 85 = 2000 HP
~7 + 0.66 = 18 HP/5
~275 + 44 = 1000 MP
~7 + 0.55 = 16.6 MP/5
~55 + 3.5 = 110 AD
~0.66 + 2.5% = 0.95 AS
~17 + 3.5 = 75 armor
30 + 0 = 30 MR (ranged)
30 + 1.25 = 52.5 MR (melee)
In most situations, the tank/attack/magic differences you see at levels 1 and 2 are specifically because of runes and masteries, with the occasional ability steroid thrown in the mix.
Otherwise Miss Fortune has more HP than Shen due to how similar base stats tend to be.
Almost always durable champions get improved HP/5 from their kit for the early game experience before they can afford durability or are truely durable. Because of the endurance race that is early-game, HP/5 can often make them feel tanky by itself.
Also, in regards to Xin'Zhao's base AD - Xin basically has 3 auto-attack steroids. He has %armor penetration, %AS, and bonus AD for 3 hits.
This is also flat out good design to have base stats be nearly identical.
If you give, say, Xin'Zhao an ability that increases the damage of each auto-attack by 15/30/45/60/75 but then throw his base AD off of averages;
those numbers are no longer universally understood.
With ~55 + 3.5 = 110 AD scaling on most champions - 10 AD means 10 AD. Getting 10 AD from a steroid ability means you got 10 AD from a steroid ability.
If you started the champion with 10 less AD, then you're saying the ability essentially does nothing. If you start the champion with 10 more AD - the ability offers a much weaker boost in overall power. The same is true for stats acquired with items, runes, and masteries.
Also - in terms of offense and defensive base stats, it's important to consider how stats scale in LoL.
Take 3 champions:
an average (A) champion with 1000 HP and 100 AD.
A defensive (D) champion with 2000 HP and 50 AD.
An offensive (O) champion with 500 HP and 200 AD.
All 3 are perfectly balanced blow-for-blow against one another. It will take 20 attacks for (A) to beat (D), and 20 attacks for (D) to beat (A). It will take 5 attacks for (A) to beat (O), and 5 attacks for (O) to beat (A). It will take 10 attacks for O and D to kill one-another.
Give them a choice of 1 item that offers 10 AD or 100 HP and things get weird though.
Grabbing the 1000 HP tank item on the defensive champion will only increase their HP by 50%, from 2000 to 3000. Against the average champion, they'll get taken down to 33% HP, 2000 damage with 1000 HP remaining, before killing their foe.
However, grab the 100 AD item on the defensive champion - and you just tripled their AD from 50 to 150. Against the average champion, they'll now kill them in 7 attacks instead of 20, taking only 700 damage instead of 2000, keeping 2300 HP rather than 1000.
The inverse is true for offense - with 200 AD another 100 isn't that useful, but with 500 HP getting 1000 will triple you're overall power.
Because of how base scale, it's considerably better to put the durability/offense/magic pulls for a champion in the ratios of their abilities, rather than their base stats themselves.
On Ryze - if you want to nerf his early-game, hitting his ability ratios or base values would be a better path to make him weaker early game than hitting his bases.