I think you can measure under achievement by level of support contrasted with success.
Last season United were the 27th best supported team in the country. All the teams above them have won trophies since 1925 with the exception of Palace, Hull and Brighton*. However all 3 of those have got to a cup final since we last did so in 1936. Reading (in position 33) are the highest club not to have won a trophy or been to a cup final since 1936, but given that their relative success is a thing of the last 15 years, I think we can discount them. You then have to go down to Bristol City in position 43 and then Brentford in 45 and Bournemouth in 46.
There really is no contest.
*Of course, current attendance rankings are a bit artificial. I have no doubt if they were competing at the same level United would have bigger crowds than Palace, Hull and Brighton.