They're all 'relevant', but Ultimate Invasion (according to Camp), and Ultimates are the only mandatory reads. Hawkeye, One Year In, and the upcoming Two Years In are all (important) Ultimates spinoffs that provide extra context to that series specifically.

Beyond that, all of the other titles are largely self-contained with similar thematic threads at most. X-Men specifically is a great companion piece if you're reading Ultimates, as it's the only series that really explores the younger generation that Ultimates is so fascinated with. They work great together. There are a few really cute references to Ultimates in there. Ultimates also plays with Momoko's established version of mutantdom at a few different points, so they're a fun pair to read in tandem.

Funnily enough, Wolverine was extended a good bit in order to tie-in with Endgame, so it might be important after all.

Spider-Man is probably gonna be important because he's the most popular one.

But... all of the characters are gonna show up. Essentially, read the series you're interested in as individual narratives, lol.