With no scaling, damage items, or auto attacks, he does up to 1010 + 13% max health damage.
That is landing both parts of sapling and a fully charged ult.
For reference, the lowest base health at level 18 (Wiki health page) is mini-Gnar at 1645.
1645 x 0.13=213.85
So his full engage is 1223.85.
Just for fun, let us throw in Gnar's 30 base MR (since that is the standard anyways), which is about 23% reduced damage.
1223.85 x 0.23... = 282.426... ; 1223.85 - 282.426 = 941.42.
For further reference:
Support:
Sona's Q->AA+Passive combo at level 18 does 568.8 base damage. 918.8 with ult. so while she deals less per-hit, in 2-3 rotations she will out-damage Maokai. She also brings a lot more utility than him.
Janna does 545 with W and a fully charged tornado and 40 AD while the shield holds. At 5 seconds and 0.625 AS, she can get in 3 AA before it dissipates, meaning up to another 120, for a total of 665.
Bard with 60 chimes deals 470 damage.
Average: 684.6
The letter "A":
Aatrox: Who is expected to build AD deals 865 base damage with all skills (1 instance of Blood Price).
Ahri: Who is also expected to build damage deals up to 1154 base damage.
Akali: 1140 abilities. Passive = 113 x 0.06 = 3.39. 1143.39 (1 AA). Also traditionally built damage (though caught building juggernaut recently).
Anivia: With 1 tick of ult, deals 870 damage. Enjoys 2 1.0 AP ratios when used optimally, and ult's 0.25AP ratio ticks per second.
Annie: 1 instance of Molten Shield, 1 tick of Tibbers DoT, no AA's. 995 base damage. Also expected to build damage and has high ratios.
Brand: 1 instance of ult: up to 1118.75. 3 instances of ult: 1818.75. +8% max health. Gnar: 1645x0.08=131.6. Total: 1950.35.
Average: 1162.9566666...
ADCs:
Corki: Full Gatling gun, 1 regular ult, 1 tick of regular valkyre: 812. Full Gatling gun, empowered ult, full special delivery: 1302.
Draven: (Before even crunching numbers, I know this one isn't fair). E + full damage R = 585. 89.25 per spinning axe x 3 (max juggle possible) = 267.75. 852.75. Mind you, he'd be doing 267.75 bonus damage every auto attack in the scenario, though only 1 is assumed.
Ezreal: All abilities = 1290.
Graves: All Abilities = 1070 (No auto attacks)
Kog'Maw: No passive or barrage, target at full health: 690. Target below 25% health: 990. lvl 18, Kog's base stats let him get 7 AAs in during barrage: 140 + 14% max health. Meaning Kog deals up to 1130+14% max health. Gnar: 1645x0.14=230.3. Total: 1360.3.
Lucian: Full ult, no passive = 1920
***Sivir: 194.25. (>_>)
Average:
(without Sivir): 7795.05
(with Sivir): 3994.65
Assassins:
Evelynn: 1 instance of Q: 310 + 25% current health. Gnar: 1645x0.25=411.25. Total: 721.25.
Fizz: 989 + 8% missing health. Gnar: 1645x0.08=131.6. 1120.6.
Edit: Forgot ult's damage amp. 539x0.2=107.8+989= 1096.8 + 9.6% missing health. Gnar: 1645x0.096=157.92. Total: 1254.72
Katerina: no ult: 575. Full ult: 750. Total: 1325.
Rengar: Empowered Bola (Because AA damage isn't factored): 910. Anyone who has been insta-gibbed by Rengar can attest to how misleading this number is.
Kha'Zix: Empowered isolation, 2 instances of passive (for ult): 1186. Sorry Rengar, Kha wins the base damage contest.
Zed: Double hit on Q: 532.5 + 10% max health. With ult amp: 772.125 + 14.5% max health. Gnar: 1645x0.145=238.525. Total: 1010.65.
Talon: Full ult and double hit on W: 900. With E amp: 1035
Average: 1120.2814285714...
Other Tanks.
Alistar: 539. Riot, your nerf bat is showing. XD
Amumu: 1 Tantrum and 1 tick of despair: 829 + 3% max health. Gnar: 1645x0.03=49.35. Total: 878.35
Braum: A tank man's tank! 776 + 2.5% max health. After Passive pops, Q and AAs deal a bonus 35.2 magic damage. At 0.644 AS, he can get in 3 AAs before his passive resets = 105.6. 881.6 + 2.5% max health. Gnar: 1645x0.025=41.125. Total: 922.725.
Galio: Fully charged ult: 1260
Leona: No passive: 990
Malphite: 1 instance of brutal strikes: 945. 3 instances (max possible with base stats): 1095.
Nautilus: 1 full instance of shield damage, 1 instance of riptide: 1010. 3 instances of riptide: 1185.
Sejuani: Full Flail (base health = 2215x0.03=66.45x4=265.8): 1215.8 + 6% max health. Gnar: 1645x0.06=98.7. Total: 1314.5.
Sion: Minimum damage Q and ult: 900. Full damage Q, ult, and empowered E: 1613
Taris: No passive: 240. Fully utilized passive (8 instances): 960.
Average: 1075.7575
Juggernauts.
Darius: Abilities base: 760. Full passive (5 stacks): 135. W minimal damage at lvl 18 (141AD x 0.4 = ) : 56.4. Noxian might not applied. 951.4.
Illaoi: W+R: 350. Tentacle damage: [145 base AD] 180+232(passive scaling)+58(Q scaling) = 470. Assuming 3 tentacles (Q, 1 for W, and 1 for R) = 1410. 1760.
Malicious Metal Mordekaiser: 845 + 35% max health. Gnar: 1645x0.35=575.75. Total: 1420.75.
Susan: 540 + stacks + 75% max health (capped at 3600 damage / 4800 max health). [I've really been underestimating his ult.] Gnar: 1645x0.75=1233.75. Total: 1773.75.
Skarner: [Is this guy still considered a juggernaut, or has Riot given up on that?) Raw ability damage: 305. Q: 134x0.45= 60.3. 365.3. [Seriously. Juggernaut?]
Tahm Kench: Spitting minions: 560. Eating champions: 260 + 32% max health. Gnar: 1645x0.32=526.4. Total: 786.4.
Average:
(Without skarner) 1673.075
(with Skarner) 1019.1875
Fighters.
Diana: (double R) 1100
Fiora: Q+W=355. E: 1 200% crit (116(base AD)x2)=232. Assuming 4 vitals: 8% max health. Gnar: 1645*0.08=131.6. Total: 718.6.
Hecarim: (1 instance of Q) 970.
Irelia: (full ult) 1020. W: 75 per AA, 6 seconds at 0.665 AS gives 3 AAs = 225. Total: 1245
Jarvan IV: 940 +10% current health. Gnar: 1645x0.1=164.5. Total: 1104.5.
Jax: (Fully empowered counter strike) 930
***Kayle: Q: 260. Fury: 60 per AA, 10 seconds at 0.638 gives 6 auto attacks = 360. Total: 620.
Lee Sin: 1140+8% missing health. Gnar: 1645x0.08=131.6. Total: 1271.6. [Kit is still way too overloaded.]
Master Yi: Ha ha. No. I'm not dealing with the sated champs until ghost-hit stops warping the F out of them.
Renekton: [The good 'ol Renekton bar.] (Empowered Q and E, full ult) 2760
Riven: (Fully empowered ult) 920. Passive: 50%AD (107x0.5=53.5), 7 possible stacks in 1 rotation, 7x53.5=374.5. Total: 1294.5.
Wukong: (full ult) 1440.
Average: 1223.10909..... (Yi not included)
Average average: 1365.907812306736796536787012987
(Disclaimer: This analysis is not assumed to be 100% accurate or to perfectly represent the proper generic values of champion base damage. The following biases exist: Champions were chosen for analysis pseudo-randomly, analysis does not differentiate between AoE and single target, All % target health damage (missing, current, and max) was calculated using mini-Gnar's base max health at level 18 (the lowest in game), All stats (AD,AP,AS,health,MS,AR,MR) are assumed to be 0 unless stated otherwise, no time frame was set (some champions had more time to apply damage than others - mostly in the case of DoT effects), all abilities were only used once unless otherwise stated, and numbers with exceptionally large decimal values were rounded or cut off.)
Based on this sampling, Maokai's base damage is roughly average. Interestingly, so is Tahm's (Though in his case, Counter-play and up-time comes into question). I concede that I under-valued Susan's ult, but his damage is still sub-par without it - reinforcing his farming identity. But champions building damage will out-pace them easily. So I still make my original claim: if they build no damage, their damage is still effectively low.
Regardless, I got a bit carried away here. Do with this data what you will.