You can start by looking on fanfiction websites like Archive of Our Own or FanFiction.net. Just search for 'Warhammer' and then sort through the results by popularity or ratings. Another option is to join Warhammer fan communities on platforms like Reddit or Discord. The members there often share and recommend great fanfictions.
One great 'Warhammer 40000 Crossover Fanfiction' is the one where it crosses with Star Wars. It's really interesting to see the Imperial Guard facing off against Stormtroopers. The different combat styles and ideologies make for a great read.
One great 'Warhammer Dark Eldar Fanfiction' is 'The Darkest Soul'. It delves deep into the psyche of a Dark Eldar character, showing their struggle between their cruel nature and a glimmer of something more. The author does an amazing job of painting the dark and twisted world of the Dark Eldar.
There's also a fanfiction called 'Emperor's Will'. In this story, it delves deep into the Emperor's thoughts and plans that were perhaps left unfulfilled during the original lore. It shows how his will could have been executed differently to change the fate of the Imperium. This fanfiction is quite detailed and really makes you think about the Emperor's capabilities and intentions.
One great 'Warhammer 40k Imperial Guard Fanfiction' is 'The Last Stand of the 13th Platoon'. It really shows the tenacity and sacrifice of the Imperial Guard. The way the characters are developed makes you feel like you're right there in the thick of the battle with them.
One great 'Warhammer Fantasy Vampire Coast' fanfiction is 'The Rise of the Vampire Admiral'. It tells an exciting story about a vampire captain who rises to great power. The story has great character development and intense sea battles that really capture the essence of the Vampire Coast.
One great 'Warhammer 40k Emperor Crossover Fanfiction' is the one where the Emperor crosses over into a modern Earth setting. It shows how his powerful psychic abilities and leadership skills shock the modern world.
There are many talented authors in the fanfiction community. However, it's hard to single out the 'best' as it depends on personal taste. Some authors who are well - known for their Sanguinius fanfictions are those who have a deep understanding of the Warhammer 40k lore. They are able to weave complex stories that stay true to the character of Sanguinius.
Another good one is 'Space Marines: Redemption'. This fanfiction follows a fallen Space Marine who tries to find redemption. The story has a lot of moral dilemmas and the writer does a great job of exploring the psyche of a Space Marine who has strayed from the path of the Emperor. It's a very character - driven story that keeps you hooked from start to finish.
One great Warhammer 40k fanfiction is 'The Emperor's Gift'. It delves into the relationship between a Space Marine and a young psyker. The story is full of action and the exploration of the complex lore of the 40k universe. Another is 'The Lost Legion', which offers a unique take on a lost chapter of Space Marines and their journey to regain their honor. And 'Sister of Battle: Redemption' is also excellent, following a Sister of Battle on her path of atonement and her battles against the forces of chaos.
I think 'The Horus Heresy' series is a great place to start. It offers a deep and engaging look into the Warhammer universe's lore and characters.