Since Dr. Mundo received a nice little buff in the last Patch in terms of skills and profits from recent item changes aswell I´m hereby writing a guide on how i prefer to play him Toplane.
This is my first guide so be gentle ;P

Pro:
+Insanely Sustainable Tank
+Constant Aoe Dmg in Teamfights
+Deals actual AA-Dmg as Tank
Con:
-Grevious Wounds hurts hard
-Can get kited by certain Champs
-No Hard CC to protect Carries

Mundos high base dmg and fast attack animation make lasthitting a breeze, in case you get pressured hard you can use cleavers to lasthit since you get the full refund on kills
under turret try to activate bonus ad of masochism to increase your ad

Titanic Hydra: Very Hard DMG-Boost if the Compositions allow you to Autoattack Enemies(low amounts of cc/protection for the carries)
RylaisCrystalScepter: Underestimated item on many tanks, since the aoe slow provides protection for your teams carries
Randuin´sOmen: Great against CritScalingChampions(Jinx,Cait etc.)Bonus: Active slow for chasing/protection
Locket: Great defensive boost(active and passive) against magic damage
Liandry´sTorment: In case the enemy team decides to ignore your dmg and is very tanky
FrozenMallet:Antikitingtool
Banshee´sVeil: In case of some crucial cc on the enemies team
BannerofCommand: magicresist aura+active creepboost(epspcially vs low ad comps)
FrozenHeart: AoE slow, vs heavy attackspeed teams
Thornmail: vs teams with basically ad only
Zz´rotPortal: nice active pushing tool+ the movementspeed bonus which is immense and buffs you in nearly every fight(except baron and dragon fights)

Hybrid Penetration: Magicpenetration because all you active abilites are magic damage, Armorpenetration because you get to autoattack alot and your ad is increade by masochism
Health Runes for survivability(scaling for the lategame and flat for early since thats the part in the game you´re very fragile early)
Ms Runes in order to get to the carries faster and simply dodge alot of dmg by running