I’m only referring to enemy heroes because Pudge can technically work in any lineup. Focus shouldn’t be on your own draft but the enemies. Pudge is very solid against any hero without escape mechanism: Hook -> Rot -> Assist from a core or second support -> Kill. Alternatively you can Rot first, Hook second.
The following heroes with escape mechanism can be killed before level 6 if played properly, the other ones are better to be ganked later or with a teammate due to the higher risk of failure:
Faceless Void: Can be zoned decently with Rot, but also killable if you Hook him after Time Walk. Important: Don’t initiate with Hook, but try to get on him with Rot (most likely with help from the core or second support). When you're trying to commit with Hook, Void can easily leap away and all you did was wasting Mana. In addition you can Hook people out of Chronosphere in the later stages of the game without any problem.
Enchantress: Important: Don't waste your Hook damage while the heal is stil active! Simply chase the Enchantress with Rot untill the heal expired and try to apply your Hook damage afterwards.
Puck: Try to Hook/Gank when he used his Jaunt or when your Midlaner has reliable lockdown.
Slark: Try to bodyblock Slarks Pounce after you hooked him so he can't leap away.
There are some heroes Pudge won't be able to kill alone, yet it's not impossible to play against them (mostly tanky physical dps and/or magic immune heroes): Wraith King, Undying, Chaos Knight, Lifestealer, Lycan, Oracle, Abaddon, Bristleback, Broodmother, Huskarr, Juggernaut, Naga Siren, Slardar, Slark, Nightstalker.
However this only applies if you’re playing Pudge as Core. As a Support Pudge the enemies draft doesn’t really matter as long as your own team counters the enemy draft’s well enough. In these cases Pudge is only giving the potential to kill these heroes, but won't kill them alone in the most situations.
Until lategame items are listed chronologically (except for TP scrolls, ALWAYS BUY TP)
Starting items: Boots, Courier, Smoke, TP
Boots give you the needed Movement Speed to reach the Offlaner with Rot. Courier so the other support doesn’t have to buy everything, and Smoke to gank Mid at Level 2 if that’s possible depending on enemy midlaner. Due to the now cheaper TP I'm usually starting with one aswell. If you're getting low HP on lane where you'd usually regen (Tango/Flask) on lane, I'm simply killing myself ending up with less money spent for a better service (full HP/Mana). Before you're dying always try to buy items. If you're missing 10 gold for your next Item (new TP/Smoke/Wards) simply wait that time so you're not losing too much gold. In some situations you might be able to help your other lanes with an early TP securing your allies life or a dead body of an enemy.
Smokes and Wards are Pudge’s best friends, always buy them when they’re in stock. The game becomes much easier with vision (applies for any hero apparently).
Build-Up: Tranquil Boots, Soul Ring, Magic Stick, Wind Lace
Tranquil Boots solve any Health/Regeneration problems, Soul Ring ensures you have enough Mana and can be used to help you suicide with Rot. You can buy Magic Stick depending on the game, but I wouldn’t recommend upgrading this to a Magic Wand, as you’ll quickly run out of inventory slots and you might have to sell it.
Why I'm personally not buying an urn: earlier when the Dismember wasn't regening your HP when eating creeps an urn was the only way to get all the regeneration you needed on Pudge. Apparently it's not really worth it anymore as you can simply use your Soul Ring and start chewing on some random creeps ending up with quite decent HP outcome. Additionally im running out of itemslots quite fast these days. Therefor buying an urn (which is not too cheap as Pudge) to sell it later on is nothing you would possibly want.
Midgame: Blink, Aether Lens,
Blink provides the exact mobility that Pudge requires; Cover long distances quickly to accomplish Blink->Dismember initiations, and great synergy with Aether Lens. Which in turn gives Pudge Health and Mana regeneration by dropping it, using Soul Ring and picking it back up to gain some fancy additional Mana. In addition it lets you Hook and blink further, increases Pudge damage output and his possibility to Dismember from longer range. Which will be very useful in numerous situations. I rarely buy Force Staff - as Blink is exactly what my Pudge needs - but that doesn’t necessarily make Force Staff a bad item. It leads to another style of Pudge play which is not suiting me too well.
Ghost Scepter, Ether Blade: Gives you the potential to survive Physical DPS, but also some decent burst using Eblade+Hook / Eblade+Dismember. However Eblade is not only an offensive item, as it can be casted on friendly heroes when they’re being focused (preferably not when the opponents still have all their nukes up). Additionally, Eblade provides very useful Armor, as Pudge has very low base Armor.
Aghanim’s Scepter: Basically a nightmare for the enemy team, ridiculous uptime of Hooks combined with Aether Lens should result in a lot of kills.
Boots of Travel/BoT2: Important to get pickoffs quickly by rotating in the lategame.
Shiva’s Guard: Pudge has little armor, provides additional Mana pool, and is good against Physical damage Cores, slows!
Scythe of Vyse: Good lockdown to transition into Dismember, i.e. against Slark, Anti-Mage, Storm Spirit, Puck, Ember Spirit (Generally evasive heroes that might react before you get your Dismember off)
The following items might be useful situationally but I'm picking them up rarely:
Assault Cuirass, Abyssal Blade, Octarine Core: attack speed not as bad as many would suspect due to the strength provided by Flesh Heap.
Bonus: Most games are either won or lost once you get Aghanim’s/BoTs/Blink/Aether Lens/Ghost Scepter, as you generally won’t get/need much (more) farm and I'm mainly focusing on Support Pudge.
The early level in Rot is much more effective in most cases, due to the most offlaners underestimating the slow coming out of Pudge. Hook is only situationally skilled at level one, for example when you smoke into enemy jungle and might miss a kill elseway.
The Point in Flesh heap at level 9 is a matter of preference and depends on the game. If you get some decent number of Flesh Heap stacks (they are even generated before you skill it and get applied when you're skilling Flesh Heap) it’s worth to spend a point to increase your hitpoints earlier.
Pudge is basically the nicest hero in Dota, as hes versatile as ****, always strong and has a huge influence on the entire game, without requiring too much XP or Gold. He is significantly underrated by many players, due to them facing people humiliating the hero by playing it awfuly due to their unawareness of his real beauty. There is no way to counter a properly played Pudge entirely. The enemies might try to limit the danger by grouping up, playing together and having good wards. The most important thing is that you are comfortable with the hero itself, have a decent game sense and track your enemies thoughts instinctively.