What version of the game did you feel like the state of Shadow was at it's best? The state being how well it performed, how close the concept came to being what you think it should be, and any other considerations you want to take into account.
That one two day period at the start on cata when our shadow mastery increase our dmg by 25%. And then we got the Nerf bat immediately. Was pretty funny how we instantly got a Nerf but warriors can one shot everyone in pvp and its been over a month.
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I really enjoyed the utility model of well, almost all of TBC. VE was a powerful group heal, even up through ICC or whenever it was converted to its lower %. Also, wearing Frozen Shadoweave for 3 full content tiers was kind of funny. I didn't much care whether I was topping meters if I was bringing consistent utility.
DKs spreading DP was fun.
Ignoring game mechanics with VE pre nerf was fun. ICC with zonewide buff was nuts.
I never raided as Shadow in vanilla, but as far as early TBC and on goes I would definitely say right now. FDCL and DI making our rotation more dynamic is wonderful, and I really like how Shadow plays when we have enough targets to use Mind Sear over Mind Flay.