Comparison table
Base (Max) Disguise Backstab
Hitpoints Kit Icon On backstab
Knife 125 (185) Yes Yes Disguise drops
Enemy screams (crit-death)
Icicle 125 (185) Yes No Disguise drops
Leaves behind ice statue (crit-death)
YER 125 (185) No No Disguise as the enemy, silently
Corpse cloaks
Kunai 70 (180) Yes Yes Disguise drops
Enemy screams (crit-death)
Health is stolen from enemy
Earner 100 (150) Yes Yes Disguise drops
Enemy screams (crit-death)
Cloak is restored
Knife / Sharp Dresser
The normal knife and Sharp Dresser are the most balanced of the knives.
They are the easiest knives to use when you need to sap enemy sentry guns. Not only can you choose your disguise, but you can sometimes take up to 3 non-crit hits from an Engineer's wrench before dying... more if they're using the Jag.
Icicle
The icicle works mostly like the normal knife, except that it leaves behind ice statues instead of regular ragdolls, alerting enemies to the presence of a spy. If a spy holding the icicle is hit by a flame attack, or pulls out the icicle while on fire, they will not burn, extinguish any flames they have, and become immune to fire damage for 2 seconds. This disables the icicle for 5 seconds, but not the spy's other weapons, allowing them to escape or to pull out a revolver to take down the enemy who set them on fire. Usually it's best to use this item if you know there will be at least one pyro spychecking for the other team, so you can make a hasty retreat. Clever pryos, however, will equip a shotgun and kill you anyway, so best to keep running.
Your Eternal Reward / Wanga Prick
The Eternal Reward and Wanga Prick are very good for killing several enemies in succession, such as those pushing a cart that are not looking behind them.
It also makes a Stab and Sap on an Engineer and his sentry gun considerably easier, provided you already have a disguise.
Be aware that you cannot disguise as your own team with these knives!
Note: You disguise only if the enemy dies from the backstab. This means you will not disguise if they: turn when you kill them and die a normal death, are ubered, have a Razorback, or are using a Dead Ringer.
Conniver's Kunai
With 70 health, you are a one-hit kill to any non-Scout melee weapon. Most ranged weapons will also kill you in one hit.
Because the health you gain is capped at 180, you actually have less health than a fully overhealed Spy with the Knife or YER (they would have 185).
The Kunai may be good for escaping due to the extra health, but it requires that you get a backstab first.
Big Earner
With 100 health, your survivability is a bit higher than with the Kunai, but a single rocket, grenade, or scattergun/shotgun shot may kill you if their pellet spread is right.
The 30% cloak you gain from using this knife is not a lot in the grand scheme of things. For the Invisibility Watch, collecting a small ammo box restores 25% cloak, a medium ammo box restores 50%, and a large ammo box 100%. For the Dead Ringer, collecting a small ammo box restores 25% cloak, a medium ammo or large ammo box restores 45%.
The Cloak and Dagger would be the watch that gains the most from this, but it's also the watch that recharges if you stand still while cloaked...
What I use
I use the Knife or Your Eternal Reward as appropriate to the situation:
- CP push/PL defense (RED): Start with YER (Saharan Spy Set) and Cloak & Dagger. Switch to Knife and Dead Ringer as appropriate.
- CP push/PL offense (BLU): Start with Knife and Dead Ringer, disguised as BLU's Soldier or Heavy. This is to draw the enemy Sniper's fire, or set off the demoman's sticky trap. Switch to YER and C&D as appropriate.
- PLR: Varies. Usually YER and C&D.
- CP symmetric / koth: Varies. Usually Knife and DR.
- Arena: That's a trick question. I rarely play Spy in arena.
Best Answer
I have hundred of hours play time with the Spy mostly using the classic knife and I was really interested to try the Conniver's Kunai as soon as it has been released.
Unfortunately, the 60 health points limit is really low and with the high level of missiles, pipes and bullets spam that there is in Team Fortress 2 it is really difficult to sneak behind the enemy lines without accidentally dying.
In the end I tested the Conniver a lot and found only a few maps and situations where its efficiency (K/D-ratio) is comparable to the standard knife. These are usually large maps, like, for example, pl_badwater (red team), cp_badlands and cp_coldfront when you are pushed by enemy team and mostly defending.