This topic is pretty old and quiet but it's not too late to reply right? :DDDD
Detective Conan was already aired in my country for some time during my childhood (2000s) so I already know the background of the anime. But one day when I was 10, I decided to watch from the first episode of Det. Conan. I was surprised with the old artstyle and the totally low quality (good streaming sites don't exist then) but I kept watching anyway. As a child, of course I would be awed about the complicated tricks and puzzles the anime provided.
As I grew up I still follow the series and with more information about the anime leaked out, I dare to point out the reasons I will still put Det. Conan as one of my top anime.
If you think they're wrong, fight me
1. The anime uses real fact and logic.
MUST SEE! --> Gosho Aoyama Interview video [link]
Aside from the anime often broke the laws of physics, this anime uses legit facts in their reasoning. Gosho Aoyama actually researched the facts in a wide range of aspects for the content of his manga/anime. Besides that, he also tests the tricks for the murders (idk how many times tho) before he actually writes them in his manga/anime so NO ONE has the right to say that his tricks in the anime are 'not logic'. Anyone remember when Conan and Reiko dialed the phone using their voices in movie 12? It actually works in real life! I wish I remember where to find the video though...
2. The artstyle
I'm not sure about other long-running anime but I'm sure none has a gradual change in artstyle as Det.Conan. We know some long-running anime like Pokemon and Naruto that stick to one artstyle in a period of time before changing it (or maybe the change is not obvious enough, I'm probably wrong). I find it fascinating to see the change of the art slowly change as I watch the whole series.
3. The connection and complexity of the plot.
Det. Conan fans know how Gosho always make the plot for an arc complicated. Especially in the current arc in the manga that connects with the old cases we won't expect to appear (don't worry no spoilers). I wish I could explain in detail but then there would be spoilers and I forgot most of the details because I'm currently watching Pokemon anime.
4. The balance
I'm not sure if I'm looking this too deep but I feel there's a certain balance in Det.Conan. Like Vermouth who is the antagonist but is protective of Shinichi and Ran, proving that she is not heartless. Or Mouri who is a drunkard but is a really caring father to Ran. I don't enjoy watching anime if I could predict what a character does next and go "Oh, of course he does it because he's like that." I won't say that the characters have realistic personalities but I enjoy seeing two sides of a character that make us understand them better.
Last but not least...
5. This awesome fan-base. (except those immature ones, including hard-core shippers)
I admit that I haven't watch or been in Det.Conan community for a while but I really enjoyed my past experience in the Det.Conan fandom. I enjoy reading threads about fan-theories and some theories really amazed me. I mean, the possibilities are endless and it's fun to throw out opinions about a never-ending series. Besides, I'm glad that I don't see too much hate comments regarding 'new Det.Conan vs old Det.Conan' so I think this fandom is the best one I've been so far.
Yeah... these are my reasons to love Det.Conan.