Can a cloaked spy be detected by watching the network graph

Certain classes are better against certain other classes.

In general: Heavy is weak against Spy, Sniper, and Demoman.

Heavy is strong against Scout, Pyro, and Medic.

Heavy is even (more or less) against Soldier.

An Engineer versus Heavy depends entirely on if the Engy has a level 2-3 sentry or not. A mini-sentry or level 1 sentry will almost always see the Heavy win at full health. In this case, the Engineer should flee. If he was building mini-sentries, building them as you flee is an excellent idea, as it will slow the Heavy down to kill it, or make him ignore it to chase you, dealing him damage all the while.

Medic is weak against Scout, Pyro, and Soldier... well, most of the classes in the game, but those three in particular.

Medic is strong against the Spy, simply because a Medic that continually moves will generally outrun or dodge a Spy's backstab.

The thing is... a Scout has the best chance of killing a Medic if he can manage to get behind the Medic, simply because a medic is a 2-shot kill with the standard Scattergun at close range.

Now, the best defense against a Heavy/Medic pair is to work with your own teammates. If you can take out one of the two, the other teammates may be able to take out the other.

If you really do need to combat them alone, Sniper is likely your best bet. As tempting as it is to go after the Medic first, if he keeps moving, he can generally outrun your scope. However, a fully charged headshot will one-shot the Heavy.

If they Uber, run, unless you're an Engineer trying to keep a sentry nest up. In this case, repair repair repair, and hope that they aren't smart enough to have the Medic block the sentry's knockback effect. If you can get a Pyro to help you by airblasting the Medic or Heavy away, even better.

P.S. NEVER go head-to-head versus a Heavy, he will always likely win simply because of how much damage he does.

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.