The general consensus is that NMB keyboards are the best, ALPS is a close runner-up and Chicony, while still excellent by any laptop standards, lags behind the other two in quality (tactile feedback, noise, etc). This post discusses English (U.S.) language keyboards only.
Chicony FRU 39T7178
ALPS FRU 39T7118
NMB FRU 39T0958
When you order a Thinkpad, you cannot specify which keyboard; it's luck-of-the-draw which one you get. If you call IBM/Lenovo to order an NMB keyboard and give them the part number you can' t order an NMB. You will be sent one of the other two kinds in its place. Lately, IBM/Lenovo has been putting NMB part number stickers over the Chicony part number stickers to essentially "re-brand" the keyboards. You can tell if you have one of these keyboards by looking closely at the back of the keyboard. If you see a white sticker over another one, you're a victim.
There is also an easy way to tell which keyboard you have without opening up your Thinkpad. First, here's an overall comparison:
CLICK TO ENLARGE PICTURES
Back side of all three keyboards:

Front edge of all three keyboards:

Chicony - backslash springs - top left to bottom right
ALPS - forwardslash springs - top right to bottom left
NMB - blue pad springs
The keyboards look identical from the top. But if you take a paper clip and gently lift up the front edge of the down-arrow or right-arrow at the bottom right of your keyboard, you'll see either a back-slash (top-left to bottom-right) if it's a Chicony, forwad-slash (top-right to bottom-left) if it's an ALPS, or blue circular pads if it's an NMB.
You can't accurately compare the typing feel of the keyboards without using them installed correctly on a Thinkpad. There is a big difference between the feeling and sound of the keys when the loose part is sitting alone vs being installed in a unit.
After spending solid software development time on all three keyboards (multiple samples of Chicony and ALPS, just one NMB), my personal opinion is:
Chicony - Good - Grade B-. They're kind of mushy, with just decent tactile feedback. Between several Chicony keyboards there is inconsistent feel. Some are more "clacky" sounding than others. I have one that is very quiet. The mouse buttons are very good, better than the ALPS. Most people report the worst typing speed is on the Chicony and I agree.
ALPS - Solidly good - Grade B+. ALPS keyboards have a consistent quality. ALPS keyboards feel very good with a stronger feedback than the Chicony. My only complaints with this keyboards is that the mouse buttons, over time, seem to develop a "squeakyness" to them. If you hold a button down and mouse the pressure point around your finger, you can hear (and feel) some sticky travel. Also, the larger keys (space, shift) tend to make more noise than the rest of the keys.
NMB - Excellent - Grade A. I can type all day on the NMB and enjoy every minute. It's at least as good as, if not better than, the NMB on the T40 series. I'm not giving the NMB an A+ because older Thinkpads, 600 series for example, had even better keyboards. NMB's are quiet, solid, have fantastic feedback and give the machine a feeling of world-class quality.
I purchased my NMB keyboard from an upgrade website after getting a tip from someone else on this forum. Now that Chicony's are being re-branded as NMB, it may not be possible to find them anymore with a lot of digging. Two E-bay sellers I contacted had posted ads for NMB keyboards but, after asking, they were actually rebranded Chicony's.
I want to close this post by emphasizing that all three keyboards, even Chicony, are WAY better than pretty much every other keyboard I've seen on other notebook computers. I shelved an almost-new Dell 8600 because my fingers would get caught momentarily between the keys. The best non-Thinkpad notebook keyboard I've used is probably a Sony, and I would give it a grade of D+. With non-Thinkpads, I would actually travel with a wireless full-size keyboard in my luggage to avoid using the keyboard on the notebook. If you have a Chicony, be happy that you have one of the best notebook keyboards in the industry. If you've got an ALPS, even better. NMB? Remove it and send it to me right away - I want a spare in case someone steals my T60p.
(Edited one time to fix a broken link to the second image)
(Edited second time to add these two Edited comments)