My strategies for battles aren't really advanced, so I will try to point out the strengths and weaknesses of each Dark type.
Umbreon
Advantages: Good Hit Points, Defense, Special Defense
Disadvantages: Bad Attack, Special Attack, and Speed
Murkrow
Advantages: Good Speed, Average Attack and Special Attack
Disadvantages: Bad Defense and Special Defense
Sneasel
Advantages: Good Speed, Special Defense and Attack, Weavile has Special Attack potential
Disadvantages: Bad Defense, Worse Special Attack. Although Weavile might have good Special Attack, nothing is certain
Houndoom
Advantages: Good Attack, Speed, and Special Attack
Disadvantages: Bad Defense
Tyranitar
Advantages: Everything but Speed
Disadvantages: Bad Speed
Mightyena
Advantages: Average everything
Disadvantages: Average everything
Shiftry
Advantages: Good Attack, Speed and Special Attack
Disadvantages: Low Defense and Special Defense
Sableye
Advantages: No weaknesses, average everything
Disadvantages: Bad Speed
Sharpedo
Advantages: High Attack, Speed and Special Attack (rare for Dark types)
Disadvantages: Poor Defense
Cacturne
Advantages: High Hit Points, good Special Attack and Attack
Disadvantages: Slow, poor Defense
Crawdaunt
Advantages: Good almost everything, awesome Attack
Disadvantages: Bad Special Defense, slow
Absol
Advantages: Good Attack, average Special Attack and Speed
Disadvantages: Lower Defense and Special Defense
I only really based this on Stats, because I am to lazy to check all those movelists. If you want a pure Dark type, pick Umbreon over Mightyena or Absol, but for hybrids, it depends on which types you would find useful. I would pick Houndoom, but that is only my preference for Fire types talking.;229;