Our best manager over the past two decades? There have been 5 of them. 6 if you include Unsworth.
Koeman lasted one season. Smith was dire, Allardyce thankfully appears to have only been a temporary hire. That leaves you with a choice of Moyes or Martinez.
To make a statement like yours, we would have to have had an absolute tonne of managers or had so many successful ones, it would make the choice of which one was the best exceedingly difficult.
Moyes did a decent job on little money for 5 years. Nothing more. Silva or Fonseca might well turn out to be rubbish but they remain untested. You can't just claim they are going to be a new version of Martinez because you seem him a failure. They are unproven but Moyes provides no surprises. We no exactly what we will get and now, just as then, it won't be good enough.
And don't give me that 'all he needed was money nonsense'. He built a good enough team to compete between 07 and 09 (which is to his credit). The limiting factor wasn't budget. It was him. HE shut up shop and ended up losing points against the big teams. HE never even tried to win away against the big teams. HE cultivated a bottlers mentality of fear towards Liverpool. HE insisted on playing the likes of Hibbert, Neville and Osman no matter what.
Lots of mistakes have been made subsequently but he is responsible for our long term decline almost as much as Kenwright. Plus he walked out on us when given a sniff of a job he's d9ne nothing to deserve despite criticising players for doing similar. The mere fact he is hailed as a hero is indicative of our great decline.