The Ultimate Universe has had some cross-overs in it's time, like Spider-Men and Ultimate Power, as well as some lesser known ones like the time that Reed discovered the Zombie-verse or him and Tony discovered the world on fire and took that Universe' Tony's head and kept it in a box, or even the time-travel story to the alternate Post-Ultimatum future where it was Sue instead of Reed that became evil.
This poll strictly focus' on Spider-Men and Ultimate Power, since they actually had their own mini-series focusing on them. Which was your favorite between the two? Spider-Men had the first ever 616/Ultimate Team-Up between two very different Spider-Men and included one of the best villains in Ultimate Spidey's Rogues Gallery who ended up actually being the well-known Mysterio from 616. It had lots of emotional moments and was strongly focused on Spidey and his closely related status quo. On the other hand Ultimate Power, continuity and timeline placement issues aside, was more large-scale and involved all the major players of the UU as well as the awesome Supremeverse. Ironically both cross-overs resulted in at least one character being stuck in the opposite universe (Fury/Zarda in Ultimate Power and Mysterio in Spider-Men, which I can't wait for an update on this in a future arc of Ultimate Spidey or even Spider-Men 2, I might add). Ultimate Power was also the closest at the time to a 616/Ultimate encounter, or at least being aware of each other: When the original Squadron Supreme Universe was brought into the battle, one of the characters (a Skrull woman I believe) mentions that they once encountered Captain America and the Avengers when they visited yet another alternate universe, which was 616. It may have been a second-hand sort of cross-over but it was the closest there was until Spider-Men.