I've never gotten a nuke, but I've gotten a 52 killstreak on third-person gamemode and a lot of the time it really comes down to when you throw a stun grenade and when to reload
ump45 silenced and a revolver, marathon, cold-blooded pro, and ninja is my favourite stealth class.
tar21 fmj and spas-12 foregrip, scavenger pro, stopping power pro, and commando pro otherwise.
A really fun loadout is a barrett or intervention with an acog scope and the usual sleight of hand, stopping power, and ninja. it's more practical on smaller maps and close quarters than a sniper with a normal scope