Sunrace CNM54 Shift Chain - 6-7sp - Grey

Pros

Excellent price for a replacement chain. Long enough for most bikes. Inexpensive enough to replace often enough so that you don't wear down your sprockets. Inexpensive enough that if you need a longer chain, you can just get two of these and link them together with your trusty chain tool.

Cons

Not really a big con, I don't think, but you need a chain tool to put it together and take it apart. But if you are messing with chains at all, you need a chain tool anyhow.

There may be higher quality chains (for more money), maybe? But how could one know?

Other Thoughts

As a chain ages it wears, and each link lengthens. (Called chain stretch apparently.) This leads to imprecise shifting and wearing down of the gears (chainrings and cogs if you prefer).

If you are new to chain replacement, you may want to read on and also probably consult some of the fine resources online or possibly at your local library or bookstore:

Is my chain worn out?: You can measure - 1" to the link or pair or links, I forget - and there is some standard for how much stretch is OK and how much is too much, I forget, look it up. My opinion is just get a chain checker, which you just hook onto the chain and it will tell you if it is time to replace, very easy.

Speeds: That is the number of cogs on your rear wheel. This is confusing, as normally if one thinks of, say, a 21 speed, that means 3 gears in front and 7 in the rear. But for chain purposes, this would be called a 7 speed.

Length: You should size your chain for your bike. One way is to use the same number of links as is on the old chain. That assumes that the old chain was properly sized in the first place for the bike as it is right now. If so, great, count them up. But to size from scratch, one method is to wrap the new chain around the biggest gear in back and biggest in front, but not through the deuraillers. Pull tight. See where they meet. Add to that length one inch (2 links, I think) and then just enough more so you can actually put the chain back together. (If one end has a hole, the other end needs a rivet, if you get my drift.)

Chain tool - you need one to size a chain, and for many chains just to put it on and take it off the bike. (This happens to be one of them.)