Bad question to ask a Fantasy-lover because it's impossible to choose just one so:
Guards! Guards! I don't think I need to say much about Discworld...and the city-watch is my favourite.
Dawnthief: I just love The Chronicles of the Raven a lot. I stumbled over the series when I was getting a bit tired of fantasy, especially the 'shining heroes go on a quest'-type and that was just a breath of fresh air. It is not free from flaws and I guess James Barclay's 2nd book-series (Ascendants of Estoria) is objectively better but the Raven-books have a special place in my heart.
Rivers of London: Peter Grant is one of my favourite main characters of all time and Nightingale is just awesome.
Mara und der Feuerbringer: fantasy-writers love mythology...mainly Greek/Roman and occasionally Celtic. Having one going for Germanic is a nice change. Plus a main-character who is snarky without being mean and who actually feels like a 13 year old is awesome.
Harry Potter: Do I have to say anything? PoA is my favourite
Des Teufels Maskerade: Talking otters and emotionally repressed Barons fighting supernatural evil during the final years of the Austria-Hungarian empire. That one sentence already contains about 326457 things that make me go SQUEEEEEEEEEEEE XD