The best wine with lamb truly depends on what you are serving it with or how you are serving.
If serving the lamb as in a tagine with saffron and spices and raisins, try to pair the wine with a rust but full bodied Côte Roti (from the Northern Rhone valley). These wines are Syrah based and have plenty of ripe black and red fruits with powerful tannins to match the intensity of the lamb and spices. Alternatively, an elegant Hermitage would still have the power but more finesse for this dish.
If serving the lamb with a cool mint yogurt sauce, try a less powerful, but still aromatic, Beaujolais Cru from Morgon or Moulin-a-Vent. These medium-bodied wines have plenty of red berry notes with earthy depths that can nicely compliment the lamb and creamy herb sauce without over-powering.
If the lamb is going to be thinly sliced and served with roasted vegetables and a fairly simple sauce, look to Italy for a Barbera d’Asti or even a Chianti Classico. These wines both have a great herbal aroma to go with the seasoned lamb, and the red fruits and acidity will contrast nicely with the gamey-notes of the meat. Barbera will have a touch more body and depth, the Chianti Classico will be lighter and more rustic.