"Longest running" might be a better, more accurate, and some might say, more impressive moniker.
Though, that is somewhat of a joke also.
The problem that occurred in Dec 2015 was due to the fact that for the first 5 years that slush ran, they didn't have any proper statistical analysis of their mining.
So when something bad happened with a miner running their own untested software on the pool, it meant that slush just continued to lose blocks and pretend there was no problem, since they didn't even understand how to identify the problem, until Organofcorti told them there was something wrong - after I pointed out the excessive statistical improbability of what happened:
https://bitcointalksearch.org/topic/m.13482822He also helped them do their current 'proof' they have, but alas people still don't understand that it's not a proof of hash rate, it's a proof of minimum hash rate.
So while you can correctly say they are the 'longest running' pool, they ran blind for the first 5 years and had no idea what was going on.
In that scenario, my pool is possibly the longest running pool with a proper statistical analysis of the mining, since I've been doing it from the start in 2014
Though I've no idea if any of the large pools understand how to do useful statistical analysis of the mining, or if they do, how long they have been doing it. They certainly don't claim to do it or supply any such information.
But anyway, who cares if slush didn't know what they were doing for the first 5 years
... and in case it wasn't obvious, while they let it go on for at least 28 blocks, they should have known 100% that the problem existed at 14 blocks.
Also, Organofcorti pointed out an interesting test that would have picked the particular problem up at around 0.42 of a single block (that I added to my testing from then on)