I've found WhatToMine to be pretty accurate - there is some variance as people pointed out with tx fees and pool fees, but beyond that it tracks well.  Nicehash is a bit tricky, as their listing on their is also an estimate based on them coin-hopping, so one something like SHA256 it's probably more accurate than something like equihash where the field of coins is a bit more even.  I haven't used antpool in a long time, but back in the day it used to be PPS, whereas Nicehash is PPLNS, so that would also affect your returns as you will have more 'luck' playing a factor with PPLNS.

The biggest thing to keep in mind with WhatToMine, especially with alts, is the market volume - some small alts can show awesome returns, but the volume is like $1000, so even if you mined it, it would be difficult to sell in any meaningful amount.  Beyond that I think it's a great tool...