What’s your favorite part(s)/moment(s) of a playthrough or the game in general?
Mine is probably things like the following:
unlocking my action skill and using it the first time
finding my first useful gun that isn’t a freakin’ pistol 
that moment somewhere between level 5 and 20 when you feel like you have tasted the flavor of your chosen character and understand it’s general purpose/potential
Definitely when you unlock the cap of a skill tree and first try it.
First legendary drop for that character.
getting your coop team out of a major scrape through good tactics.
Beating the game
enough skill points for 2 level caps
putting together some extreme build of gear + skill points + circumstances successfully. Bloodsploding Pete for example.
Doing something in the game that you know would be really tough in a different circumstance but you made it easy. Example: I was helping a team beat an arena. We were down to one guy in the arena. I threw a seeking grenade to destroy the remaining turrets. Or easier example, phaselocking a flying jerk that was bugging everyone else.