For me, it was the PotC refurb. I like Cap't Jack well enough, but the photo flashes drive me nuts! And once folks start (at the first Cap't Jack), they just keep flashing away all through the ride...And everyone within 3 boats of the flasher is disturbed...which in turn gives more idiots the idea to whip out their flashes. Before the rehab it wasn't much of an issue, at least not during the day. (After midnight, there was kind of an unspoken allowance for some craziness, though most the folks then were savvy enough to not avoid flashes.)
Ooh, the 20,000 Leagues refurb is also a contender! R.I.P 20,000 Leagues! (The sting is finally starting to wear off as the memory fades....)
Can I list an attraction that is so desperately in need of a rehab that it is painful? (even though it had one long ago) If so, I'll CoP tops my list. The last scene just hurts.
Another rehab that gives me mixed feelings is the Frozen/Malestrom rehab. Having a Frozen ride is great, but Maelstrom was an old favorite. We wish they'd budgeted to add an all-new Frozen ride instead of taking Maelstrom away. It would have been great fun it they'd added a whole Frozen-land to HS. It could have been a tiny bit like Universal's HP area perhaps....