Deus Ex is an all time favourite of mine thanks to its massive replayability, and The Nameless Mod captures all the best elements of Deus Ex into a neat new story with plenty of humour and some gameplay elements tweaked quite nicely. But for pure silliness I highly recommend the Malkavian Mod; it involves killing a potted plant called Hitler using Manderly's hunting rifle along with a mission to get pills. And make sure you promise not to kill Filben's wolf.
Human Revoloution is also an excellent addiiton to the series and found it highly enjoyable, but Invisible War felt like it was disconnected from the rest of the series, it had some solid elements such as choosing which factions to help, but it does have the issue of having a very tough act to follow and the overly simplified gameplay that makes it feel more like an FPS rather than an RPG, just lacked the more open world feel to the original.
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Falling is not defeat. Not getting up is.