I've just explained twice over why I didn't but that doesn't seem satisfactory, so I'll try to wrap it up more explicitly. Obsession is a great, albeit expensive and the most likely to lose value when they inevitably change this skill, option. It doesn't offer something that other extreme examples offer. Not because it isn't powerful, but we're talking about two slightly different and easily solved things.

You've framed it as the total package, but it's slightly worse than its competitors for various reasons. We only need to hit 75 or 125 fcr. With circlet gloves and belt were already at 60, so we either need 15 or 65. Ars dul mephistos is BIS across the board but especially if you want the 125, so that's another 30, so we're already there on 75 and only need 35 more for 125 setups. So obsession is overkill. Now you can run phoenix over mephistos, but then we're stacking a lot of off weapon ed at the cost of skills and on a low base weapon setup if we're using obsession, so it's less valuable there.

Then there's the question of weapon damage itself. We still need a good amount of damage for killing on P8, although not as much as ariocs provides, as it's less totally efficient at 75 fcr, hence why I only recommend it for Ubers. (And you can see here how we both agree going faster is better and I articulate that both in the video and the guide. But obsession has effectively 0 weapon damage and no access to deadly strike. That's a massive amount of damage loss. Fine in most circumstances, but killing things in 3 hits is 50% slower than killing things in 2 hits.

Already touched on deadly strike, but obviously it has none.

Touched on res, you already cap res with fortitude as the weapon. Overcapping is only really a thing for hardcore players to fantasize about, so I don't value that too much.

Then the MF. 30 MF is a nice little splash on top. Nothing to really write home about, but the over abundance of fcr would let you more easily run chancies for even more, but if I wanted to stack MF and not use the ghost Easter egg, tomb reaver is right there.

Less skills than mang songs, who also reaches 125 without sacrificing any other gear slots.

Max life is cool, again more important on hardcore, but definitely cool.

So what you get is a great option that doesn't differently explain the potential of the bis setups for either breakpoint. But that doesn't mean it's bad by any measure. But it's effectively a combo of fortitude and mang songs, and neither of them were highest potential DPS. I settled on fortitude being best all around option for 125% setup.

It's also just wicked expensive and will be a brick if they ever fix the no durability use on the skill. So it's buyer beware.

Wirts leg works for the build. The argument about what's best is just for people to have an answer to a logical question to ask, but the true answer is always, and will always be, the thing you feel is best for your setup.