Which disguise should I pick as Spy

As a spy, you're best off spending your time working to assassinate the opposing medic. Since the other team has only one medic, it is a huge advantage to your team if you can kill (or badly hurt) the other medic, even at the expense of your own life.

Since this is a competition-level play, you can expect full communication from the other team. You will very rarely fool them into thinking you're on their team. They all should know where everybody is. (Whether or not they really do is another matter). The simplest way for you to be effective is to find a nice sport to Cloak and Dagger, wait for the enemy medic to run by, and revolver or ambassador them from a distance.

Mix up your spots, be unpredictable, and stay hidden. You may also want to spend some time hounding the enemy sniper, depending on his skill level. I've seen highlander teams completely fall apart from a skilled enemy sniper. On the off chance he's razorbacking, the ambassador will be the weapon you want to go with. Mix your disguises, but definitely use the sniper disguise - that class matches closest with this play-style.

Lets weigh the different options:

• Revolver / Big Kill - Standard damage. Kills lower HP classes in 3-4 non-crit hits.
• Ambassador - Lower damage shots, slower firing rate, no random critical hits. Headshots do 102 damage if you haven't fired recently... you can tell because the firing reticule in the center of the screen will be at its smallest size. Kills lower HP classes in 2 hits if one is a headshot, otherwise 4-5 hits.
• L'Etranger - Lower damage shots, recharges cloak on hit. Kills lower HP classes in 4-5 non-crit hits. Extremely useful when being used with the Dead Ringer... less so, but still somewhat useful with the Invisibility Watch or Cloak and Dagger. Also part of the Saharan Spy set, in case you want the silent decloak set bonus.
• Enforcer - Higher damage shots, cloaking takes half a second longer. Kills lower HP classes in 2-3 non-crit hits. Extremely useful with the Dead Ringer, where the downside doesn't apply.
• Diamondback - Lower damage shots, no random critical hits, gets guaranteed critical hits from backstabs or when your sappers destroy buildings. Kills lower HP classes in 4-5 non-crit hits or 2 crit hits. Unless you're fighting mini-sentry Engineers, this gun is largely inferior to all the others.

These days, I tend to flip between using the Enforcer with the standard Knife/Dead Ringer and the L'Etranger with Your Eternal Reward with any cloaking device (and Saharan Spy set). Occasionally, I'll use the L'Etranger with the DR and standard Knife or the Saharan Spy set with the Dead Ringer.

In terms of pure damage, if you have time to aim, the Ambassador is your best bet. If you can't, the Enforcer is. If you tend to run out of cloak a lot, the L'Etranger can help, but remember that you break disguise when you shoot enemies with it.

In my personal experience, the L'Etranger seems to miss more than the other guns. All revolvers shoot straight down the reticule for the first shot, but somewhat randomly for successive shots until they cool down. This is much easier to tell on the Ambassador, as the cooldown is identical to the headshot cooldown.

Having said that, if you have auto-reload on, the reload animation takes slightly longer than cooldown, so letting it autoreload and complete the animation is another good way of telling if the cooldown is done.