Are you talking accurate in terms of what Disney states or accurate in terms of closest to ACTUAL wait times?
As far as closest to posted wait times, that's hands down got to be Disney's official app for Android and iOS. It's just called "Disney Parks" now, but it was formally called Mobile Magic (and still is inside the app). It actually uses Disney's systems to present wait times, FP times, etc.
As far as closest to actual wait times, I found Ride Hopper to be the most accurate for at least popular rides. The problems with those ones are that the times can quickly fluctuate and they are dependent on people entering the times in, which can lead to invalid and stale data.
Well mousewait was helpful until somehow my phone says it no longer works on there system, I emailed them to get it fixed, never got a email back. Terrible service from them, I used to love it but since they dont care about their users, i can no longer get my problem fixed. Bad show Mousewait