I'm sort of surprised Deku is topping so many people's list!
I can see why. Most of Shonen Jump was dominated by idiot protagonists for the longest time so when Deku (and Soma) came on the scene they were a breath of fresh air since they subverted expectations. Deku is a smart nerdy type who works hard to earn his power and Soma is a cocky (to a fault at times) protagonists with the skills to back up that confidence.
Anywho my list
1) Deku
2) Ochako Urakara (Gravity Girl)
3) Shoto Todokiri
4) Tsuyu Asui
5) Fumikami Tokoyami
On the record, I love almost all the characters in the series aside from Bakugou who I have been warming up to recently. Since there haven't been any major arcs apart from Deku's progression this is mostly off a character design and power set standpoint. That being said my favorite villain by far is Tomura Shigaraki. LOVE love love his design. Perhaps one of my all time favorite villain designs. His presence is just plain creepy.
Quick explanation for everyone.
1) Obvious, albeit too easy, first choice. Deku has received the most development, has one of the best powers in the series and has a great cast to bounce off of. I already highlighted above why his character is great but his power is mysterious and I am intrigued to learn more about it. The only thing I have to knock against the character is his full hero design is strictly a joke at this point and I would like a better costume for him in the future. Especially that cowl which serves to be a joke once and then is subsequently blown off and never worn again. I really enjoy him in the standard school uniforms though.
2) Another easy choice for me. Ochako is one of my favorite females in a running manga to date. She is bubbly but dead serious about wanting to become a pro hero. Meeting her equally upbeat parents says a lot to and only endears me to her character more. Her power is a unique one with tons of utility the series seems invested in proving. I love these technical fights in manga (see Shikamaru from Naruto) so I look forward to seeing how her power is used in the future. If the Bakugou fight is any indication then I can't wait what else she has in store. In terms of design I simply like her hero costume. It has a nice space suit vibe to it.
3) I didn't want this list dominated by obvious choices but here we are. Shoto is on here because he is simply OP. 2 really powerful superpowers in one guy and he has had some interesting development to boot. He skyrocketed up my list because he has had a lot of development alongside Deku and recent chapters have me frothing at the mouth in excitement to see the interactions these two get. In my head canon they become BFF's in the manga. I like his design too even if he looks like a straight copy and paste of Zuko from Avatar. But if you accidentally (or purposely) copy from anything at least it was a good design to go off of.
4) Made the list for 2 simple reasons despite having little time spent with her. Great personality and easily in the top 3 character designs in the series. Frog Girl seems to be the first to start to piece together Deku is related to All Might and further showcases her smarts in the villain invasion. Her hero costume is equally inspired and really nails that 'Toad' look. The only problem I have with her is because she has a specific power set I don't know if she can be taken further. Either way it is nice to see a Shonen manga with several notable female characters already. Now we just need a female supervillain...
5) My 'surprise' choice. He mainly makes the list for one main reason. I LOVE his design and power set. He is (almost) inexplicably a bird and his powers have interesting (and funny) effects depending on the lighting around him. Much like Ochako, I look forward to seeing his powers used in creative fights. From what little we have seen of him he seems pretty smart as well, saving his teams but in the second round of the festival. Wish I had more to say about this guy but we will have to wait and see where the series goes from here.