This is based on my experience with Dream Theater’s music and how it helped me become a better musician.
As far as technicality goes, any one of the band’s crazy instrumentals will work. Erotomania, The Dance of Eternity, and the middle section of Outcry are all masterclasses in virtuosic playing.
When it comes to being more creative, it depends on what aspect of music you’re looking to improve on to help make interesting and memorable songs. For melodies, I would go with their ballads. With simpler song structures and more steady rhythms, these songs require catchier melodies and chord progressions to hold the listener’s attention. Look at songs like Another Day, The Spirit Carries On, and Wither and listen to the way the melodies are shaped around the chords.
For rhythmic intricacies, you can’t go wrong with their mid length songs. Tracks like Take the Time, Just Let Me Breathe, and Panic Attack use polyrhythms, metric modulation, and various other rhythmic tricks to keep things fresh and interesting. Listen to these songs to help expand your rhythmic horizons.
For song structures, I would check out their mini-epics. With these songs being over 10 minutes in length, they can’t have a standard song structure otherwise they would lose the listener. Songs like Trial of Tears, Home, and Breaking All Illusions have various different sections intricately interwoven into each other while being connected by reoccurring musical themes and motifs. Figuring out what makes these songs work will almost certainly help you learn how to make unconventional song structures work which will allow for more musically complex compositions.
The band’s 20 plus minute epics are of course the perfect songs if you want to hear melody, technicality, rhythmic complexity, and intricate song structures all woven into one, but I feel that finding songs that allow you to focus on one of these ideas at a time will yield better results, as it will allow you to perfect one idea before moving on to the next. Their epics are simply a combination of everything that makes Dream Theater great.