If you know me, obvious reasons. It's been my favourite Pokemon since 2006 and I literally don't know of anyone else in the world who loves it as much as I do. I dare you to find someone. I dare you. I've described why I like it a lot in the past but at this point it just sort of transcends the Pokemon itself, if that even makes sense. Whenever I start to think "Pokemon is getting kind of boring, I won't get the next game at all", I remember that I can train another Cherrim and then I'm already excited to play. It's my top favourite so I don't think its entry in this thread is very interesting for me. Cherrim was also the first Pokemon I ever got to level 100 without cheating (aka rare candy duplication).
Before Cherrim, Pichu was my favourite. It still shows in my household because I have so much Pichu stuff. I don't collect it anymore, but I still look fondly on my merch and I still really like the Pokemon even if I don't use it (well, its evolution line) in teams anymore. People have usually hated on Pichu in the past since it was so "unnecessary" and that still makes me sad to see. It might not be my favourite Pokemon (or even favourite electric type) anymore but it will always have a special place in my heart for being #1 for so many years.
I won't even try to explain this one. It's probably my favourite Pokemon after Cherrim and I only started to like it because of an inside joke with my best friend. Not even joke, we both just stupidly thought it had a nose hair fro until someone mentioned its moustache one day and we looked at each other and went "oh my god we are so dumb" but we already liked it for a stupid reason so we still like it now. It also makes me sad how much people hate it. ;_;