I'm a fan of both source material and crossovers. With the fact that Deadpool can't enter the MCU and it's not just on his demerits alone, but also a technical issue, having him along with the Avengers would jank up the universe and may cause a collapse of said universe.

So that's
1. Avengers
2. Deadpool

Now that is put aside, I would add the Justice League and make it one of those oh-so-rare Marvel\DC crossover (Yes, they have happened, I discovered this in a factoid list that delved into The Incredible Hulk)

As Marvel once owned a superhero property belonging to Toei (Sailor Moon's Animation studio) and Warner Brothers currently owns distribution rights to Sailor Moon in the US. (DC properties are handled by Warner Brothers) I would fight to put The Sailor Guardians in. However, if this is live action especially, I want a Japanese-American cast for better representation.

I have already heard a number of white voice actors playing black parts quit in solidarity of the Black Lives Matter political movement and studios to start recasting non-white parts as response. I am on board with the concept of better representation.

But one request and it's from the top of the Sailor Moon creative team: We are not, repeat, NOT making a Sailor Earth, delegitimizing Sailor Pluto or make Exo Planet based Guardians. Unless the boss lady says yes, it's a firm no!

So...
1. The Avengers
2. Deadpool
3. The Justice League
4. The Sailor Guardians

Don't get me started on Super Sentai or Power Rangers
1. Too many teams
2. Each team in Super Sentai was in their own worlds as was much of Power Rangers with their reality.

Your ULTIMATE dream crossover.