The great thing about spy weapons right now is that EVERY single one is viable. I am writing all of this with pubs in mind. Comp weapon selection is a whole other mindset.
Stock everything is the most versatile. I highly suggest that spies spend at least 100 hours just on stock. I made the mistake of not doing that and had to spend many more hours unlearning bad habits.
The revolver is strong, the knife has no downsides, and the IW can get you anywhere you want to go and you can maintain cloak for a long time if you know how to surf ammo.
The amby is great if you have good aim. If you aren't getting those 102's though you are losing damage. Amby headshots have no limits, if you can aim you can be very dangerous.
The diamondback is now viable, it is similar to the amby in that you can get 102's but instead of rewarded for aim, you are rewarded for backstabs or sapping. If you have those crits stored up you can be scary.
L'Etranger is a great cloak upgrade. You lose a little damage, but it complements all the other watches in its own way. With the C&D it gives much more mobility. C&D spies are often ridiculed in pubs because they spend a lot of time just waiting. The L'Etranger upgrades this a little bit. With the DR it gives a little more unpredictability. Most competent players know that when you feign you will be headed for the closest ammo pack to recharge. The extra cloak time gives more area to get away and you can you to the further away ammo, or at least get further away to recharge. And it just makes the IW even better with cloak for days.
With watches, admittedly C&D is the least useful in pubs, I use it a lot on first point payload on defense.
The DR is the most controversial spy item. People call it a crutch or whatever. I started off only using the DR, then I switched to using the others and shunned it. I have recently been using the DR more in pubs getting back in practice, because there are times when it is useful to use. Part of its effectiveness is the element of surprise. If you have been using another watch for the whole round, and suddenly switch to DR, make a believable feign, then the enemy is A LOT less likely to realize that you are still alive, and you can catch them off guard.
The brings me to my last point about the DR. The 'believable feign' I think that is where most of the DR hate comes from. The fact that some spies don't use the watch as an illusion tool, but just to block damage. They will +forward to the enemy with the DR up until one little thing sets it off and then they try to get behind the lines. Or they hold it up 24/7 to make sure they never take much damage and give themselves time to escape. I wish that Valve would change it so that you can only hold it up for limited time (like 5 seconds or so) and also once you drop it you cannot bring it up right away, maybe a cooldown of 5 seconds or so. That might encourage the proper use of the weapon i.e. actually faking a death.
Finally, sappers...
If you use the Red Tape you should be punched in the gooch.