Undead vs Human
There are a 3 main options in this match. The first one is to DK harass with a Rod while doing hit and runs on his Peasant line or Creepjack as much as you can. Basically, all you want to do is harass the crap out of the Human until you finally reach Tier 3. At Tier 2 play it safe and add a Lich to your DK, it'll help with AoE. At this point add 2 Statues for healing and keep on pumping Ghouls. Tier 3, you want to add a Crypt Lord, buy an Orb of Corruption for the Lich and continue your delay until you get Destroyers and Aboms, the 2 units you'll find yourself using the most in late game.
As for him, he'll probably be setting up a Riflecaster force to push hard at Tier 2. This is the most impacting battle in the game, because it is very hard to defend against. He'll probably have an AM/MK/Footies/Riflecaster while you're defending with DK/Lich/Ghouls/Statues. You're going to need towers if he does come in to your base, but your main goal is to pick off as many units as you can without hurting yourself too much. If you can get past this and reach Tier 3, the game starts getting a lot easier because Destroyers rip through Casters and Aboms eat Riflemen alive. Watch out for Gryphons and Paladin at this point.
The second option is to go for a suicide Lich/Ghoul rush. This is one of those do-or-die things. Basically, you throw up 2 Crypts and pump Ghouls as hard as you can without teching. Once your Lich comes out, sell your TP (or keep it, if you fail the rush it's going to be hard to recover anyway) buy a Rod and bring as many Ghouls as you can. Hit his Peasants hard, Nova + Ghouls + Skeletons can rip them up if he's not defending too well. Once you hit level 2 start using Dark Ritual on Skeletons or Ghouls that are about to die to keep your Lich Nova-happy. Send all low HP Ghouls back home and reinforce yourself with high HP Ghouls. If you failed and lost a lot of Ghouls or lost your Lich, then you'll have to fight really hard to win because at Tier 2, his push will be really hard to beat.
The third option is Crypt Fiends. Usually accompanied by a Beastmaster (MaDFroG style) or DK in this matchup, you'll find yourself trying to creep as much as you can as fast as you can. You can actually fight battles in this situation unless you find yourself against a LOT of Defend footmen. At Tier 2 the Human will probably get Breakers and Casters, which is where you need to play as defensive as possible. Get 2 Statues and go straight to Tier 3 where you will add Destroyers, a Crypt Lord or Naga and Aboms. However, if you scout and see that he is low on units and that your army can defeat his, then put up another Crypt and break 50 food. DK/Lich or Beastmaster/Naga as well as Fiends/Stats can lead to a quick win.
Undead vs Orc
The lifelong discussion in this matchup is whether to go Ghouls or Fiends. Basically, Ghouls means that you will be harassing most of the game trying to make sure the Farseer does not get too many levels so that later on you don't get owned in Chainwave. Fiends means that you need to creep as fast as possible while avoiding direct confrontation with the Orc as much as possible. You can't afford to fight against Speed Scrolled Grunts.
Using Ghouls, find yourself resurrecting the Death Knight from the dead early on. He's an excellent way to steal creep kills early on as well as provide healing and mobility for your Ghouls. Pick up a Rod of Necromancy and try to tech to Tier 3 as quick as you can. Bring along a few Ghouls after your tech and creepjack as much as possible but avoid actually fighting him, as Chain Lightning rips through Ghouls. Once Tier 2 comes, he'll most likely go Wind Riders. There are two options you have and they are going for Destroyers really quickly, or a mix of Fiends and Ghouls. Since you started with Ghouls, you don't have the option of straight Fiends because the transition would be really rough.
Fast Destroyers is really hard to do and you're going to need towers if you want a chance against it, as well as putting up your Slaughterhouse near the back. Basically, it means the Orc will waltz over to your base with a FS/TC/Grunts/Wyvern along with maybe a Heal Scroll. This means you will base camp as much as possible until Tier 3 when you can research the Destroyer upgrade. The best second hero in this situation is Lich because the AoE really helps vs. air. In this case you are going to need armour upgrades to survive against Bats, usually 2 is enough.
Fiend/Ghoul is a great strat and there is an excellent replay that demonstrates it here.
Basically you start off with Ghouls harassing as much as possible. If he runs away try your best to surround whatever you can. Basically you go straight for Destroyers, but in Tier 2 you put up a second Crypt and a delayed Slaughterhouse. This means less Destroyers at Tier 3 but a stronger Tier 2 army. Research Web and get Fiends out ASAP, delay his Wyverns. If you find yourself in your base with 2 or 3 Fiends while he has 5 or 6 Wyvern, put up a Spirit Tower just in case. Dreadlord works well in this situation because it provides leech for your Ghouls vs. Chainwave and the ability to sleep the TC.
Going straight Fiends also works, and you have 2 hero choices for this. Death Knight, which is the standard works well for Fiends because it provides healing and mobility. Crypt Lord is also a good choice if you go Beetles/Impale since it provides a great way to creep early on as well as the ability to fight battles.
Undead vs Night Elf
Against the Nightelves you have many strategies to use and to counter with. Basically, the Elf will try to hit you early on and mid-game while stopping you from levelling your powerful heroes. The best first hero in this matchup is indeed the Death Knight, who has probably the best spell to creepjack with as well as sniping low HP Archers and DotTs.
Unfortunately for you, the Elf has many heroes to choose from first. Beastmaster, Demon Hunter, and Warden are all very popular first heroes that have high damage output and excellent spells against your army. The Beastmaster is particularly dangerous because your main form of Dispel comes at Tier 3 in the form of Destroyers. This means that if you let your opponent get a high level Beastmaster, you're going to be rushing for Destroyers. There is absolutely no way you can fight off that many summons without Dispel. However, if he chooses Demon Hunter then he will most likely harass you early game. This is where you must learn to hide your DK while creeping and use Ghouls to try and surround the DH. You also have the choice of just mass Coiling the DH, sending it over to heal up in Moon Wells while you creep. Finally, the Warden is a great harasser and has some very powerful AoE against Ghouls and Gargoyles. You have to avoid head on fights unless you are certain you can nuke the Warden to hell, or unless you have a huge amount of heal scrolls. Second heroes are usually Panda, Firelord or KotG. The Panda has been nerfed a bit and is a high-hp AoE spewing hero. Meaning you need mass Heal Scrolls, or Abom/Destroyers to fight against this guy. KotG is useless since all his spells are negated by Destroyers, just nuke him. Firelord works like a Beastmaster but is much more fragile. It boasts potentially dangerous summons that you should try and kill/Dispel first.
Basically, the best strategy against Night Elf is Ghoul/Garg -> Abom/Destroyer. It gives you a high edge on Archers early on and a great harassing unit, the Gargoyle. The main thing you have to remember against Elf is not to fight head on. Early on, you can use Ghouls vs. Archers but against Hunts you'll find yourself suffering. A dual Crypt Ghoul rush works really well in some cases when the enemy only went Archers. AoE and Summons are way too dangerous in most cases unless you have Heal Scrolls or Tier 3. At Tier 2, pump Gargs from 2 Crypts and decide on whether or not you will get yourself a Lich. Solo DK works well vs. NE because level 3 Coil is really dangerous to Archers and DotTs. Watch a few MaDFroG/CantChat. replays and watch how he uses Gargs because they are probably your best chance in living the Tier 2 push.
Fiends also work well but you have to watch for Faerie Fire. If possible, get 2 Destroyers along with your Fiend/Statues so that you can eat up Faerie Fire. Watch Philbot replays if you'd like to know how to use Fiends in this matchup.
Destroyers/Aboms/Orb of Corruption come at Tier 3 and they are what you need to actually be able to fight head on battles without 2 Heal Scrolls and 800 mana worth of Nukes. Grab a Cryptlord or Lich if you want at this stage because that extra AoE really helps against DotTs and Archers. Finally, just remember that if you made it this far through delaying and harassing, then chances are the Elf is completely frustrated and vulnerable. Harass, delay, prevent expansions.