who's the hardest to kill in a straight up fight at the extremely early levels.
Base stats are incredibly close among champions, to the point Miss Fortune actually has more HP than Shen.
I'd look at defensive abilities pretty much exclusively for determining this.
Also - for going full-tank strats, it's important for a champion to be impossible to ignore despite being a brick wall. Which generally comes from 1 of 2 things: Amzing crowd-control lockdown that forces enemies to deal with you (Nautilus), or amazing damage for tanky DPS (Nasus).
A very large number of durable champions have health regeneration as their main durability capacity at early levels before items come into play, allowing that Cho'Gath, Garen, Nasus, Maokai, Volibear, Udyr, Warwick, Aatrox, Renekton, Irelia, Alistar, Taric, and more to actually feel durable in the endurance race of lanning without much in the way of tank stats.
Regeneration is something most of them have, so you'd need to get crazy to win there.
I'd look elsewhere for defense. Like, enough defense to actually initiate fights in season 2 levels of support income (non-existant), where you're an item behind everyone else.
Leona and Alistar were basically it for that type of tank. Leona gets her mega-tank steroid at level 3, Alistar gets his at 6. If you're looking at level ~3, Leona is the most durable champion at that point in the game, for a 0/30/0 supportive champion.
Udyr and Warwick probably the most durable champions at level 3 for 1v1 purposes. Because being able to clear the jungle with no leash or runes or masteries is something they can easily do.
Also solo Baron with, like, 2 items.
It's more drain-tank tactics in those cases over being really tough, but still. You transition into mega-tank with both of those and still eat people's faces off.