Wow, this was a harder question than I thought it would be, once I put some thought into it! For your first ever MK ride I could see where a classic like Dumbo or teacups would be fun, but then again at age 26 you’ve probably already been on something similar somewhere else. Pirates and Haunted Mansion are both very solid choices, but I am hoping you are rope-dropping MK (that way you can catch the welcome show Let the Magic Begin!) and idk, my preference wouldn’t be to do Pirates or HM before 10am... I need to be fully awake to fully appreciate them! Macabre at 8am just isn’t my scene, although don’t get me wrong,
I adore both rides. I was going to say Splash Mountain, which is our first ride at MK most days, but we actually bring flip-flops to wear on Splash and then change back into sneakers after. The risk of soggy socks is too great, especially if it’s the first ride of the day. You should definitely ride Splash, it’s my favorite ride in all of MK, but I do recommend bringing flip flops or spare socks if you can.
Which means that my recommendation for first ever Disney ride is... Seven Dwarfs Mine Train! It’s perfect, the coaster part is great but it’s also a dark ride which is really unique. And it’s kind of fitting since Snow White was the first Disney movie, now this can be your first Disney ride! Congrats on your trip, I hope you have an amazing time!!!