I do agree with many of the comments in this thread but wonder about the significance of key individuals over a strong squad. We (I am a Spurs supporter) have Harry Kane a wonderfully skilled striker but just like Van Dijk in defence for Liverpool it is not enough if they get injured. Strength in depth over superstars? Having it all would be nice as did Man City with all their money but even that did not last. Then there is the anomaly that is / was Leicester City.
It is intriguing what makes a successful squad to win the EPL the balance of strong players in every area across the pitch, back up squad players and of course the coach, backroom staff and leadership at the top level. Not an easy recipe to concoct.
Fair play Liverpool you smashed it this year.
I would argue that it is neither "key players" nor a strong squad, but a mindset that encourages the players to view themselves as a team, a core part of a collective whole, so that they play for the team, rather than for themselves.
Thus, I think that it is a mixture of key individuals in certain specific positions, who are encouraged to see themselves as part of a team; in other words, in a truly successful team, key players are not allowed to view themselves as "superstars" and insist that the team be built around their needs, or to serve them, but that they themselves - irrespective of how gifted and talented they are - are a part, a cog, in a supremely functioning whole.
Having said that, while you need a strong squad, there are certain positions - I would submit that goalie is one - where a top team - or a team with ambitions to be a top team - must have an outstanding player in that position.
While van Dijk was massively important for Liverpool, until they sorted out the goalkeeper position, and recruited somebody genuinely excellent - which they did with the arrival of Alison - as a team, they still fell short of consistent excellence.
Re Spurs, I think that they have some superb players in some positions, but are stretched too thinly as a squad; in truth, Pochettino - who was an excellent manager - brought them far further than they could reasonably have expected given the resources at their disposal, (after all, they had several top four finishes in the Premiership, and reached the CL final) and made it clear over a number of years that the club needed more strength in depth and that the squad needed additional players, which meant a greater outlay on the part of the owners. Actually, I think that Pochettino brought Spurs about as far as he possibly could, and further progress could only come with greater investment in the team which the owners were not willing to contemplate or undertake.
Leicester were a classic case where the sum of the whole was greater than the sum of the individual parts (though some of them were very talented), and where the players played with passion and belief for each other and for the team. They were also fortunate that - for a variety of reasons - the year they won the Premiership none of the teams likely to challenge them were a completed project, all were some sort of a work-in-progress, or had unaddressed or unsolved problems. However, that in no way takes away from Leicester's stunning achievement, they seized their opportunity, and - from Christmas, were never below second place, and led from the front for weeks before they were crowed champions as worthy winners.
Re strength in depth, such strength in depth is usually the result of serious strategic thought on the part of a gifted and thoughtful manager. Moreover, the creation and crafting of a team that manages to achieve this, takes the very best managers (Ferguson, Klopp, Guardiola, early Wenger, etc) a few years; this is not done overnight.
Actually, they usually take a year or so to bed themselves in, to take the time to study their own team, to persuade the team to want to improve, want to change, want to become better, and they also study the opposition, and to think about what conditions - training, diet, etc (and what specific players, specific to that team's particular needs, not just a "superstar" who happens to be for sale), what experts, what staff, are required to bring about a substantial, or significant improvement.
