Hello everyone, welcome to my build! :)
I love Udyr for many reasons such as he's a great jungling champion. He is also very tanky and can kill enemy champions if built the right way.

I chose :
-Greater Mark of Desolation for early armor penetration while jungling.
-Greater Seal of Alacrity for some extra attack speed.
-Greater Glyph of Warding for magic resist.
-Greater Quintessence of Desolation for armor penetration also.

For masteries, I chose 3/17/10
Armor + magic resist early game so creeps/monsters doesn't do so much damage on you and for 30% longer lasting buff.

If the game last long enough for you to buy your last item, sell Wriggle's Lantern and buy an item depending on your situation.

Situational Items (Damage)
Bloodthirster - if you're doing fine and you want some more damage & lifesteal.
Madred's Bloodrazor - if you're against heavy HP tank such as Cho'Gath/Singed/Mundo and etc.
Black Cleaver - if you're against heavy armor tanks such as Rammus/Nasus/Taric and etc.
Executioner's Calling - ONLY get this item if you're against insane-healing enemy champions such
as Sona/Soraka/Vladimir/Mundo and etc.

Situational Items (Defense)
Randuin's Omen - a good defense item with an unique active skill.
Sunfire Cape - get this early game if you're under-farmed. A great items which gives some more
armor and health.
Thornmail - get this if theres Tryndamere/Gangplank/Master Yi and Pantheon (if he is fed, if
Pantheon is doing badly, then don't get it).
Quichsilver Sash - get this item if enemy has just too much crowd control.
Hexdrinker - Against Vladimir/Karthus ultimate. It also gives some attack damage and magic
resist.
Guardian Angel - You can grab Guardian Angel instead of Hexdrinker.

I'll update whenever possible and please, don't just look at the items and character stats and then downvote. At least read through it or try it before downvoting. Thank You.