For a 2000 pt game you get to choose 4 special units. You could just opt out of the decision and have one of each but that might not be the right choice. Let's take a closer look.
Starting with the lowest points cost unit and working upwards...
Tomb Guard (12 pts)
This unit must be fielded in a unit size from 10-25. The profile is very similar to Black Orcs with slightly better initiative (+1) and slightly worse Ws (-1). Black Orcs have choppas and Tomb Guard have Tomb Blades (magical + killing blow !).
All in all and considering the army factors of immunity to psychology, cause fear and unbreakable I would consider Tomb Guard to be reasonably good value for points spent. Not as hard hitting as Black Orcs (choppas) but overall more resilient and a better tactical 'bet'
Given that skeletons will almost inevitably lose their combats and gradually turn to dust I would have to say that you should field at least 1 large 25 models Tomb Guard unit. Also, you should include a hero and/or the Tomb King and magical items to ensure they are capable of winning combat rounds and benefitting from their Fear.
Tomb Guard may still need some flankers to support them in negating the enemy's rank bonus.
Ushabti (65 pts)
These undead constructs are enemy unit routers. With a strength of 6, Ws 4 and 3 attacks they are going to do some damage. However, with no ward save and a low (beastie) toughness of 4 they are very prone and expensive considering this. Once in combat however they are lethal but your enemy's plan will be to avoid fighting them altogether. If your enemy is a fast mover or has very strong missile troops I would be wary of talking Ushabti into battle (so don't use them against Wood Elves or Bretonnians !)
If you do use them you will need a 'missile shield' unit (or cannon fodder if facing Empire, Skaven or Dwarfs !)
They would be excellent against slower armies or those without too many +2 str great weapons in the rank and file.
Don't send them to fight the enemy hard men, use them against rank and file for the victory points. They are one of the few units in your Tomb King army that is a genuine victory point grabber, the skeleton units wear down the enemy, these smash them apart !
Carrion (24 pts)
Rathe large undead vultures are important if you need to take out or just tie up missile
troops to allow you Ushabti to get across the battlefield unmolested. They are essential if you are facing 'artillery strong' armies. If neither of the above apply then don't bother with them because they are not battle winners.
Tomb Scorpion (85 pts)
This giant creature represents excellent value for 85pts. Compare it to a Tree Kin Elder for the same cost. The scorpion has +2 Move and +1 Wound. The treekin is a forest spirit with a 5+ ward save and 4+ 'bark' save. The scorpion has less defence (5+ armour save) but is far deadlier with poisoned attacks (if you roll a 6 to hit then no to wound roll, just roll armour and ward saves) AND killing blow (6's to wound = a kill !)!
The only problem with the scorp is that it's a lonesome solitary bug :) so unit size is 1. However, it has a special attribute...
'It came from below' This rule allows the scorp to remain buried in the sand - anywhere you want on the Tabletop (if it's not very sandy then we can only presume that scorpy is buried in the mud !). As each turn passes it becomes increasingly likely that the scorpion will emerge (D6 roll of 4+ on turn 2 decreasing by 1 each turn there after). You do have to roll a scatter and artillery dice to see where the scorp arrives (a hit on the arty dice means it turned up where expected).
I look on this 'It came from below' rule as a major bonus. Place the marker in the middle of the enemies deployment zone and you'll probably end up charging him as soon as you shake the sand off your claws and dust down your stinger.
So in summary...
Tomb Guard - Yes, at least 1 unit
Scorpion - Yes, at least 1
Ushabti - maybe
Carrion - only for engaging long range foe or for tying up closer range missile troops.
