Thinking about this more, and I genuinely have to wonder which is going to be the highest selling film of Phase 5. Guardians V3, or Deadpool 3.
Guardians will be the finale of the current Guardians set, and the last major James Gunn endeavor with the current cast and crew. But Deadpool 3 is going to be the first time that Wolverine has been properly billed and advertised in any form of film material, since Logan in 2017 (excluding X-Men Apocalypse), and Deadpool 2 in 2018. A 7 year gap and a 6 year gap respectively for those characters.
The case can be made for either; even with the R-Rating most likely going to stick for Deadpool 3. It'll probably usurp IT Part 1; but I wonder how much higher it can go.