What lock or locks would you recommend to a NYC rider?

the conventional wisdom:

kryptonite evolution or new york locks
sheldon brown locking method
kryptoflex cable through the front wheel

don't leave your bike outside overnight
don't leave your bike outside for an extended period of time

locks don't prevent theft, they only slow it down.

example: i never lock up my road bike. i will use my track bike for errands, since it has no component group to steal, so i will lock that to a parking meter or something but never for more than an hour.

I'll add:

Try not to park your bike where you can't see it.

It's not just the lock that matters. Replace all quick release skewers with bolts: wheel, saddle, lights, etc. Use Zip Ties on smaller things that can be easily removed or take them with you. As said above, all these measures won't prevent theft, they'll just slow it down. So it's a combination of the locking device being used, how long the bike is left alone and the location. My bike is not super duper, but I would not consider leaving it outside for more than maybe half an hour. But I always try to bring my bike inside stores, offices, etc. Or, if I can, I find a crowded bike rack and stick my bike somewhere in the middle A lot of stores and offices I go to have lobbies or delivery areas and they don't mind if I leave my bike in the there.

I also have a motion sensing alarm on it

And don't hope any of this will prevent it from getting stolen. Not even parking it outside a police station or in eyesight of a doorman or under camera surveillance (no one cares). And even all this under one hour. Just be ready to get it stolen.

Get a cheap bike and insurance. Don't get attached to your bike. Have a backup plan for the following days without a bike. Subway/walking/cabs will be your friend until you have a new bike, ready to be ridden and...

...get stolen.

This may sound bitter (and it is somewhat) but eventually you have to accept it as part of living in the city.

LoJack

I should start a LoJack For Bikes business, it'd be like printing money in NYC. Anyone got a few million to invest?

I've lived here 18 years and have never had a bike stolen. I have a Brompton folder which never gets locked; always comes inside with me. And my 2 nice road bikes get locked on the street occasionally – never for more than an hour or so. If my only option was to ride a cheap crappy bike, I wouldn't ride at all.

If (when?) your bike eventually gets stolen, give the description and serial number to your local bike shops and ask them to keep an eye out for it. I lucked out big time this way in Boston. The thief/buyer/recipient came into the nearest shop to get the bike's rear wheel trued from where they'd pried off the U-lock. This was the very same day I told them about it! The shop called me, and I bought pizza for all of them.

It's important to get a cable that has a separate lock on it (not the kind that just has two loops that you'd just loop through the U-lock) and lock it independently to the structure (ideally, a separate point on the structure) in addition to looping it through your frame and the U-lock. The tools that can cut cable locks are different than the ones that can break U-locks, and most thieves will carry one tool and target bikes locked with the type of lock that their tool can break. Make sure you choose a sturdy structure to lock to that can't be easily destroyed or disassembled! Sign posts are a bad idea, and test parking meters and bike locking posts by pulling up on them and twisting first to make sure they aren't loose.

And never underestimate the importance of "uglifying" your bike.

Now that you've said that, you'd better be extra careful not to jinx yourself. I'd keep one hand on that folder at all times if I were you.

Makes sense but didn't help.

I forgot where I saw the video but someone showed how easy it was to steal a bike in the city in broad daylight in some of the busiest streets which confirmed for me that leaving a bike locked isn't the best idea. I'd go the route of a foldable bike I can take with me. In the video no one even bothered to look as the thief used a hack saw, a blow torch and even a small circular saw.

If you care about everything you see while moving in the city, you go bananas.
Folding bike may be an option most of the time but even that you can't take everywhere. Space is limited here. You can cloak entire shops with a folding bike.

This is the most important advice. All methods can be defeated with time, but like any other random crime, it's all about opportunity--try not to present it. The more public the place you chain your bike the better. The trouble is that at bike racks folks have an excuse to be messing with bikes, so locks are still important. This is why I don't ride anything to work that I couldn't stand to lose. There are always stories of folks who step over dead bodies or shut their windows while someone is being killed, but it is not the norm.

i park my bike outside overnight everyday for the past 6 months no issues. I have pinhead skewers on everything. With that said, I know i'm tempting fate. it's an entry level model and teh way I see it, it's paid for itself 3 times over by what i've saved on cab fare. I recently upgraded to a new road bike using some parts from this bike. I really need to keep this bike inside and get my other bike back together as my daily commuter. One reason why my otherone has been fine for 6 months is that is a flat bar roadbike so little value in the shifters and things like that. PResently this road bike someone could easily undo my stem bolts and cut the cables to my brifters and take off with a nice set of brifters and handlebars. fortunately they are microshift so not super expensive but it would still be an ass pain.