Usually my favorite part is leveling my people are not really paying attention, then checking in like 5lvs and seeing someone with a 30% growth rate like gaining speed every level. Watching my 20%str growth rate mage have the same Str as Mag and being very confused.
In the same vein restarting your save using the same / different people to see how consistent they were, or how widely different they could be.
My favorite part is when I see my enemy composition doing what they should, like the strategy I designed for the enemy works as intended. Another thing I really like is seeing how effortless it is to combine for kills; how I can use different units at different parts of the game and consistently have the exact/ a bit more damage for kills.
If I’m honest, hacking is very stressful to me (so is splicing lol) but the pay off of seeing it all come together is the best part for me, especially when it comes to making sure certain events don’t totally break the game. Also in a similar vein to Zarg, god it’s so satisfying when enemies do what you want them to do, oh and when green units finally behave like they’re supposed to and don’t eat hand axes to the face. I also quite enjoy testing different strategies sometimes to change things up and sometimes just mess around for fun and get surprised. (Ie. The Jagen somehow getting a perfect level up)
I’m looking for bugs
Testing how many cheese strategy can be done and overall enjoying the challenge I make for myself (I playtest my game using underleveled units and some of the worst possible screwed stats, works perfectly to see if the chapter is fair or not as well).
Testing the game’s stability so everyone can play normally ;p
Suffering
Always satisfying to see an idea come to life. Figuring out the right map flow and unit balance while being able to execute the vision I have is fun.
I do dislike when I make changes to the early part of the game and then want to replay to make a save for later chapters - makes testing a bit of a chore at times. I’m also a hacking noob so I don’t know how to do autolevels or get to specific chapters for testing without a save, so I end up replaying early chapters a ton, which makes the process less fun overall when I’m trying to make progress.
ftfy
My favourite part is when I play the whole chapter and nothing breaks.
The best part of playtesting your own chapters is when you’ve finished playing through a fun chapter. Because you know, you just played through a fun chapter.
My favorite part is the feeling of complete disbelief when you make a ridiculously complicated event, and get it to work in the first try 
Here, I improved it further. As we all know, nobody hates Fire Emblem more than Fire Emblem fans.
But seriously, I like watching my cool cutscenes/events work out properly, as well as the general fun of playing a FE game tailor-made to my preferred gameplay style.
It’s very fun to see your own idea and design for a chapter come to fruition as you playtest it.
But as much fun as that is, it’s actually more fun for me personally when a chapter plays differently than I suspected or intended it to, in a good way. Maybe it was a mistake that the armorslayer mercenary charges me, but I find it adds another layer of urgency to the chapter. Or perhaps that forest tile that I put down without a thought hampers a brigand just enough so that he doesn’t reach a village too quickly for my liking, when actually playing the chapter.
Of course, accidents harmful to the map are more common than those that are helpful. But it makes those that are helpful all the more welcome.
fire emblem bad
elebits
laughtrack, upvote, whatever.
I enjoy gameplay that is challenging but forgiving. The stakes are never too high: you can pretty much always salvage your situation or retreat and try again with your slight bit more exp. It feels bad to outright lose a chapter and have to reset, so I try to avoid that.
when a talk event gives me a death screech
When your abitrary allocated stats by chance allow some big brain strategies.
“Hmm, maybe I should change the starting positions a bit. Okay, now time to play the chapter.”
“Wait, I should move that enemy a few spaces closer so he can attack you if you attack that other guy. Alright, now I’ll play the whole chapter.”
“I gave that shaman a hand axe instead of a tome, how did I even do that? Ok, I’ll go play the whole chapter now.”
“Shit, I didn’t make that item droppable. Okay, NOW I’ll-”
Too real 