Addi needles are made of brass. Their "Turbo" and "Sock Rocket" lines are further plated with nickel. The "Turbo Lace" line is not nickel-plated, but has the hollow brass tips coated with some kind of protective resin. If you are nickel-allergic, these may be an option for you.
KnitPicks metal needles are made of nickel-plated brass.
ChiaoGoo and Hiya Hiya make stainless steel needles, with no plating or coating. These are marginally less slippery than nickel-plated needles.
Those old-school pearly grey metal needles by Inox and Prym are made of (powder-coated) aluminium. These are the least slick of the metal needles.
Needles tarnish rather easily, especially the nickel-plated ones. It's mostly due to the oils in your hands, and gets especially noticeable if you let the work sit for a time. I find that wiping the tarnished needles with a microfiber cleaning cloth (like I use on my glasses) is usually enough to get them shiny again. If not, use a mild degreaser like vinegar. Windex and a paper towel works as well.
[tarnished Ni-plated needle. It's just dirty.]
Nickel plating is done by a process called "electroplating", which requires the underlying substrate (brass, usually) to be super-clean. If there's any grease or dirt around during the manufacturing process, the electroplating won't adhere properly and can flake off later. It is really annoying, the flakes end up in your knitting and the needles get noticeably textured. The underlying brass layer shows up, see the photo below. Cheaper needles (like KnitPicks) have a higher rate of plating failure than the more expensive ones. If your needles exhibit flaking, call customer support. They'll send you a replacement.
[needle showing plating flaking off]
Powdercoating is tough as nails, and I've never had any of my needles so much as show a chip. Even keeping them clean is hardly required.