I've been playing a lot of Spy lately, and I have to admit, he's a lot of fun. I'm not very good, but I still have a ton of fun.
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When i played TF2 spy was my main. With the buff to the Your Eternal Reward it was really fun, i used that and the L'Etranger . I used to use the Ambassador, but it got nerfed..
IMO spy is one of the funnest classes, because if you are a good disguiser you can have a lot of fun with the enemy team.