I’m currently finishing setting up a 5x2x2 pvc enclosure from kages and looking to get my first snake (and expand it to 6x2x2 if needed). I really love the idea of getting a pine or bull snake- I’m confident I could work past any displays one puts up when they get nervous, the idea of that sort of challenge sounds really fulfilling. I’m interested in getting an intelligent adult snake that will be engaging to handle and enjoy an enriching enclosure with many hides. I’m also considering a Mexican black kingsnake, I love their coloration and am unafraid of one bitting me by mistake or in general, I figure if I own a snake I’ll probably, eventually, get bitten though hopefully very rarely. I know these animals have different needs, but I’m stuck between getting an MBK and a leucistic bull or pine snake. I really like the idea of a slightly bigger, potentially more challenging (for a beginner) and engaging snake that’s fairly hardy in terms of its environment, which is why bull and pine snakes appeal to me. I know I want a non-tropical, fairly hardy colubrid that gets between 5’ and 6’ that is curious about its surroundings. Is working to earn the trust of an MBK as rewarding as with a bull or pine snake in your opinions? Any advice on which I should get? Thank you in advance!
I don’t have much experience with these species but @eorr53 has experience with the bulls and pines, good luck in getting your first snake!
I know bullsnake isn’t a MBK nor a black pine. But I don’t own a black pine, and I haven’t had MBKs for very long. The bullsnake is in the same genus, and sort of the same size. It is my favorite snake.
However for you? The only way I know for you to know with certainty is to get both!
Neither one is going to inflict a devastating bite, and if you get your pet young, you’ll have the opportunity to get to know it as it gets larger. If you are really intent on getting an adult, try to look for one that you can purchase in person. This will give you the opportunity to interact with it along side the seller, who can help you better understand how to handle the animal.
For what it is worth, I think the bulls likely get the largest of the snakes you mentioned. The pines are probably close behind. MBKs mature a fair bit smaller. You didn’t mention it, but black milksnakes are pretty cool as well, use similar husbandry, get to be a black adult snake, and end up around the 5 to 6 foot mark. No personal experience on black milksnakes, so if someone corrects me, believe them over me.
I’m certain from what you’ve written that you’ve put a lot of thought into this. I suspect that with a bit more contemplation you’ll decide on a path forward that is right for you.
MBKs are smaller and less messy. 
I have a young Mexican Pine male and a young female Bull snake. My Bull has a pretty significant wobble that I knew about when I purchased her but she was priced very reasonably and is the very Red Bull I wanted. She is the sweetest girl. I’ve had absolutely no issues with her at all. The wobble probably has something to do with it.
Now about my Mexican Pine! He is gonna be a handful I believe. He makes his hissy cobra stance when he thinks I’m invading his territory! He will accept his mouse, drop it and do his little hissy dance raring and hissing at me. It’s a hoot because he’s still small. I’m sure it can be unnerving to some but all I do is occupy his front half and scoop up his back half and he’s a little puppy dog! He’s never struck at me or bitten me. For now. If you want a bit of spice, a pine is an avenue to travel down. I also have a couple of Gopher snakes that display some spice but not like the Pine! A gopher could be another consideration. I think gopher snakes are extremely underrated. I love both of mine!
I had a MBK at one time, a female. She was very squirmy and a musker if I remember correctly. I took her as a favor to a friend who had too many at me time. I ended up re homing her. Since she was still pretty young I cannot say what she would have been like as an adult. However my pine was and is way more spicy than her.
You say you want an adult but does that mean now or eventually grow up a pine or bull or MBK?If I were you I wouldn’t start out with an adult pine or bull just because I think it will be easier to adjust and grow with the temperament of a baby/juvenile, especially if you are a first time owner. You don’t want to start out with a snake that could overwhelm you from the get go. Just my opinion/
Goof luck with your choice! I would like to know what you decide on btw…..