Lots of medics with lots of ubers on high-damage-dealing classes. Demomen, if they are any good, usually work best against a sentry farm — stickies are perfect for this job, the usual problem is living long enough to deploy and detonate enough of them.
Heavies or pyros can also be effective, if they can get close enough (short range for a heavy, point-blank for a pyro) without being overwhelmed by the knockback before they can do damage. The medic should lead the uber combo in (particularly if the partner is a heavy) — he's faster, so he can absorb the knockback better.
If at all possible, back the uber team(s) up with as much raw firepower as you can muster — the aim is to either overwhelm the engies' ability to repair stuff, take out the engies directly, or take out dispensers. Soldiers are good for this support role, as they can place well-aimed damage from behind the uber. Demos can support well, too — their indirect fire can damage engies and dispensers without having to kill the sentry first.
Sending a spy in to sap just before or during the uber(s) can be a good idea, too.
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
The main problem you're facing is that you're playing 2Fort and using a setup that doesn't work well on that map. Basically using any different watch or knife would help you get past enemies immensely easier. 2Fort is not a spy friendly map but since you asked....
Without a disguise in the rather claustrophobic map you will be killed quickly so try to get into the water to enter their base. You won't find a lot of safe DR decloak spots near their spawns. The YER works on the idea that the first stab is the hardest to get but gets easier once you have the disguise. Look for a player with his back turned in the doors near the ground level.
Alternatively stand in the corner by the enemies doors on the ground level. When an enemy drops down from above from their spawn you may be able to get a quick stab.
Just to repeat: 2Fort sucks for spies. Going DR and/or YER in 2Fort makes it harder for yourself. Switch up the watches/knives