Tenebra.
With all respect to Battman, Axem, Darius and other BP freders who put tremendous effort to make Tenebra working, I genuinely disapprove it's gameplay. Except Universal Truth and Vindicator mission. Current iteration is much better than the first, but my initial playthrough left me with dire impression.
Overcomplicated, requires several playthroughts of every mission to get familiar with their mechanics, etc. But also Tenebra is extremely unforgiving to player's mistakes and provide you with extremely tight time window to accomplish your goals. In the assasination mission, there is power-off option if you get noticed, but it also require you to fly considerable distance before you can use it. I remember my 2 takes: First I engaged it 2,5 clicks from Myrmidons chasing me and I died anyway. Second time it somehow worked, but I failed anyway because this escape mechanics took too much time. After many attempts, I finally established my way - hack the freaking Mjolnirs.
Carthage mission remain as my least favorite mission I ever played in any mod. I can summarize it with single pharse: How to play this thing?. It's a noise, where everything is happening at once, full of op Alastors and other witchery. I'm just completely lost in the chaos of battlefield. But I was playing. For the outstanding writing and story.
I marked 2nd and 3rd options. I don't like situations, when I just don't know how to follow directives to make effect. I disapprove missions freded "by authors for authors", with badly executed complex mechanics and complete lack of intuitiveness. However various examples of awesome missions with tricky, complex mechanics exist. One notable example is extraction of spy from HoL installation from BtA. Also I like hard missions, requiring me to push my dogfighting skills to my limits and fight for my life. It's okay if gameplay requre effort, but all effort must be rewarded.
I praise Solaris for it's cloak system. It's easy to use, but hard to master. Darius carefully introduces one to it's extremely powerful mechanics with simple turret strike. It provides excellent base for experiments and encourage to try different things like ambushes, risky maneuvers and other shinenigans. And unlike Tenebra, cloak applies to half of the campaign. I learn one thing and I can master it to the end of my days.
Ad meritum: Freders: Make complex things working simple way. I play for story. Lemme enjoy story :].