One story I wish I'd read earlier is 'The Weight of the Watch' over on AO3. It's not your typical action-fest, honestly. The premise hooks around Ben after the Omnitrix is finally gone, and it explores the weird psychological vacuum that creates. How do you define yourself when the thing that defined you for a decade just... vanishes? The author gets into his relationships with Gwen and Kevin in a really grounded way, less about alien fights and more about the awkward, sometimes painful process of rebuilding a normal life. It's character-driven, which is why I recommend it for new readers—it doesn't assume you've memorized every Ultimatrix transformation from season seven.
Another absolute gem, though it's a crossover, is 'Green Lantern's Ten'. It sounds wild, but it works. Ben finds a power ring instead of the Omnitrix. The story is less about the ring's powers and more about how Ben's fundamental character—that impulsive, compassionate heroism—shapes the ring's capabilities. It's a fantastic intro to fanfiction's 'what if' potential without being overwhelming, because the core of Ben is still so recognizable. The writing is clean, the pacing is tight, and it wraps up in a satisfying 20 chapters. It's a complete story, which is a huge plus when you're just dipping a toe in and don't want to commit to a 300k-word epic.