I've seen many UB builds posted here, but the latest seems to be a pure ooze build. Some of the ooze players say that you sacrifice too much to get spit as well, but I've found that a combination build can be done easily that takes the strengths of both without sacrificing much from either.
Skill Build:
1) Spit I
2) Diseased Claws I
3) Inner Beast I
4) Spit II
5) Foul Grasp I
6) Post Mortem
7) Spit III
8) Ooze I
9) Ooze II
10) Spit IV
11) Ooze III
12) Ooze IV
13) Inner Beast III
14) Inner Beast IV
15) Putrid Flow / Acclimation
16) Acclimation / Putrid Flow
17-20) Up to you from this point, I usually either get the last 2 points of Diseased Claws & 2 Stat points, or just straight 4 Stat Points
Item Build:
Favor Item: Blood of the Fallen
1) Banded Armor + TP Scroll + Combat Pot/Cap Lock/TP Scroll (depends on map)
2) Vlemish Helm
3) Boots of Speed
4) Unbreakable Boots
5) Nimoth Chest Armor / Hauberk of Life (Depends on who you are fighting)
6) Heart of Life
7) Sell Vlemish Helm, buy Narmoths Ring
This is the base item build, it will cost around 13,500 to complete. The total stats from these items (assuming Hauberk of Life) will be
+3150 HP
+60 HPS
+800 mana
+50% MPS
+10% Movespeed
After this it's up to you, Mageslayer/Ashkandor are good choices, Journeyman's Boots/Wand of Speed are also great if people keep running from you (the 50% movespeed proc is almost always on if you wade into towers/creeps).
This build starts off focusing on Spit (great harasser, can be used against buildings & to get runners/ help an ally easier), and then mid game pumps up ooze. I don't take Ooze early because I would rather focus on leveling & staying in the lane over melee w/ooze running which make you lose a lot of your hp, even if you win. You really don't want to go toe-to-toe with the big tanks if they are at full HP until you max ooze & get more of your HP items either. Heart of Life is pretty key to this build as well, it will keep you from constantly having to run to the fountain, and it gives you a full tank of Mana and a lot of HP every 30 secs. The ideal way to kill someone is to soften them up with Spits, use the HOL to get back up to full, and then charge in. If they are already low on life, just charge on in then. I don't feel that you give up anything (other than you don't become a melee terror until mid game) by taking Spit, and it's great for taking down buildings, runners & adds DPS (& AoE after lvl 15) to your normal fights (still ticks while foul grasp is going off too). It is a bit mana intensive, but late game you can play it just like a pure Ooze build if you want to save mana (I always keep enough mana to get off a Foul Grasp). A lot of people think you don't need Post Mortem either, but I find it great for farming & taking on waves of creeps (and the damage hurts buildings & other DG's in the area). This is basically an all-round build that works great all game & makes UB able to do most anything.