Deus bloody Ex, the first one. Both the tutorial and the first mission are mostly useless and many players outright drop the game during the first mission. Afterwards the game shows its true colours, but the beginning is just rough.
I think Liberty Island is a brilliant introduction, showing you how you can take multiple different approaches to achieve your goal. But yes, it's also a serious trial-by-fire. I remember I couldn't even find Filben when I first played.
It seems we agree. In the hindsight: yes, it's a decent level. As an introduction: hell no. With player not knowing what to expect and with barely any character abilities it's one of the most confusing first levels I can recall.