My top 5 from my school, in order:
1) Palgwe Chil Jang (7th Palgwe form, 12th form we learn, 2nd to last form before Black Belt). This is one of the more complex forms we have, with a lot of techniques that flow back and forth like a wave. I have a special place in my heart for this form, because of glowing praises I got from my Master after performing this during my Junior Black Belt test. The form is detailed and nuanced, but yet is one of our most beautiful forms.
2) Taebaek Hyung (18th form we learn, form for 3rd Dan test). This one I just recently learned, so maybe there's some recency bias. It's taking the place of Palgwe Sah Jang for me, our blue belt form, because in a lot of respects it's very similar to our 4th and 5th Palgwe form. However, I much prefer the pacing of this form. It ranges from slower and stronger than the Palgwe, as well as some of the combinations being faster and higher difficulty. It combines most of the best aspects of Form 4 and Form 5, and I feel a bit of pride in being one of the few people at my school who knows the form (we only have one other student who needs it yet), so maybe there's a bit of bias there, too.
3) Kibon Sah Jang (4th form we learn, form for adults to get Orange belt or kids to get Green belt). We have 5 Kibon, or basic forms, but this one is actually the hardest of the five. I know, because I made a mathematical formula to calculate the difficulty of each form, and this one ranked pretty high. Like how Taebaek Hyung combines Palgwe 4 and 5, Kibon Sah Jang combines Kibon Yi Jang and Kibon Sam Jang (Kibons 2 & 3). Our first form teaches the basic pattern, and then the next two forms sort of branch out and teach different concepts, which are combined together into Kibon Sah Jang. This form feels like the finished product of the three before it. If Kibon 1 is the frame of the car, Kibon 2 is the engine, and Kibon 3 is the cab, but Kibon 4 is the complete car.
As with the others I brought up, it has a special place in my heart, because it got me a gold medal at my first tournament as an adult, back when I was an orange belt. It is the most dynamic form we have up until Palgwe Sam Jang or Palgwe Sah Jang (Palgwe 3 or 4, our 8th or 9th form). This form feels like a return to basics, but still with enough meat to it that I can practice a few different concepts.
4) Keumgang Il Jang (17th form we learn, for intermediate ranks between 2nd Dan and 3rd Dan). This is a more traditional version of Keumgang Hyung. This is by far the most complex form I know. You know how a lot of the posts I've made before have mentioned how great I've done forms in the past or how proud I am of them? This one...I'm not proud of yet. I know it, I understand it, but my body has trouble with it. It's still a challenge for me, and I'm still working on some pieces that need a lot of polishing, like the triple side kick and the 360 hook kick. Of course, I'm tempted to ask my Master if I can just do a mirror image version of the form, to get my good side on all these techniques.
So, I don't have a prideful stake in this form, but it does tickle the other itch I have with these forms: the level of difficulty. Most forms I know don't really challenge me. I might try to work on my stances, or make my techniques more crisp, but aside from Taebaek Hyung (which I only recently learned, and already feel I have a better grasp on than Keumgang Il Jang), this is the only form I still break up into pieces or individual techniques to work on, instead of working through the whole form at once.
5) Koryo Hyung (14th form we learn, first Black Belt form). We have a Koryo Il Jang, which like Keumgang Il Jang is a more traditional, more difficult version of the Hyung form we learn first. However, I actually prefer Koryo Hyung in this case. I like the flow of it better, and even after being a black belt for 2 1/2 years, I'm still learning nuances to the form that I didn't realize before. I do Koryo Hyung better today than I did when I got my 2nd degree, and I did it better than than when I passed my 1st Dan, 1st Gup (or as I explain it to people, Black Belt v1.1) test. I also think this is the only black belt form I've taught in class, so once again it has a special place in my heart. I mentor the black belts in my class when I have a chance, but it's rare I lead the whole class through forms. So, a bit of a place in my heart, a well-paced form, and one I'm still learning details on 2 1/2 years in, Koryo Hyung is my 5th favorite.