You are not allowed to buy a single item. Which champion is strongest on base values alone?
Question in title.
earlygame and/or lategame.
Question in title.
earlygame and/or lategame.
riven
Earlygame:
because of his Passive AD-boost
because his W passive now gives him a free 30 Armor and MR from minion kills alone (plus his base dmg values are quite insane)
Lategame, every champion that can stack stats or damage from minion kills
(Q stacking)
(AP stacking)
(HP stacking)
(Arm + AP stacking)
Nasus probably wouldn't be that good. The main strength of Nasus is that he can get tons of damage from q farm while building tank so he can get to people and smash them. Without items all he's have is his ult for suitability, so he's likely to die before getting to someone.
We are talking no items here. No one has any defense other than built-in shield and passive gains like Shyv, Wukong or Garen. Nasus gets INFINITE damage farming his Q. There is no limit to the stacks.
And if you have no armor and no weapons and no bonus hp, a Guy that can deal 500hp with one attack, is VERY scary.
Plus, his survivability is not dependent on his ult (which, btw. provides him with extra hp and attack damage, I forgot about that when I included him in this list), but his passive.
Garen by far.

Without a single AD from items, Darius gets 400AD at max level with his passive. Not to mention he already has the third highest HP, HP/5, and Armor at max level.
So, probably him.
Also anyone that deals HP% damage.
Trundle's a contender, but it depends on who's on the enemy team.
For example he'd do very well against a team with a Sion on it, because he'd get all that free health and resistances.
I probably think the ones without Mana are pretty strong.
I think Vladmir would be scary, because he has sustain and no mana and can stay in lane for quite a bit.

Try it, you'll see why
Hard to say...but....

These are 2 that come to mind, but one I'm surprised I haven't seen is 
ranged slow with good damage attack speed/lifesteal steroid True damage CC immunity ult