ME1 and the Loneliness
I loved the planetary descriptions especially the more dynamic ones. My favorite planets are Agebinium, the mission with the nuclear weapon and how it tied in with the Skyllian Blitz. Its twin Maji and how pirates and slavers sent their cargo there to duel to the death before the suns dawned and boiled them away. Ontarom due to the electrical storms caused by the close proximity of its moon (loved standing there and watching the lightning in the sky while the theme "A Very Dangerous Place" looped in the background...felt awe inspiring and eerie). Amaranthine due to the violet and magenta sparkling of the sediment and the fact that a poet was inspired to write a love poem about it. Eletania because it was a beautiful place, that Prothean sphere quest (how they were watching cro-magnon man). Chasca is equally beautiful and it was a place humans were going to settle so the exploration meant something in that sense.
Overall, it ended up making the Presidium feel especially like home. It was there where everything was safe, familiar faces, familiar species, familiar architecture surrounded you, and mundane activities (shops, bars, strip clubs, casinos, restaurants, massage parlors) were booming 24/7.
ME2 Main hubs
I actually like the limited exploration of Omega, Zakera Ward, and IIium (but then again I also love when my imagination and other people's fanfiction adds to these places). I don't need to wander the entire city/district, just fill it with interesting characters, quests, chatter, advertisements, news, and shops. Each hub has its own appeal as Kasumi points out; the lights of Ilium are just for show but on Omega every place where you see a light has a life behind it.
I would prefer Zakera Ward (Shin Akiba). Zakera has a lot of different cultures colliding within it. I could imagine the sounds of different music, entertainment, voices. The different fashions and the smells of different foods from different species.
I noticed subtle things on Omega/Ilium/Zakera that added to the the relationships between different species. Outside the main entrance to afterlife you see a Turian/Human couple, the human wearing a colonist outfit. You see the same thing in Zakera, but the it is more implied on Omega. The Turian and Asari husband and wife shopping for souvenirs on Zakera, and the Quarian and Turian in Eternity, plus the bacherlor party (friendship between Human, Salarian, and Turian).
Bioware briefly allowed us to get a greater feel of Ilium (LotSb) with Liara's apartment, the skycar chase around the city, and Hotel Azure, It's not a lot but it adds to its flavor. Would have been nice to have a car chase on Omega (cannot imagine a drive like that would be easy).
ME3
I was to depressed and agitated by all the dismal crap I was hearing to take in the vistas on the Presidium; however, from Thane's location in Huerta you do get a good view of the Presidium Commons (can't see the people however).
Tuchanka. Sur'Kesh, Rannoch...out of all of them Rannoch was the one I really like as I got to sneak around cliffs and valleys and snipe geth. I felt rushed through Sur'Kesh and Tuchanka (great storyline here overall though).
For ME4 I want a 50/50. What we got in ME2 and ME1. A few city hubs with lots to do and a few lonely planets with unique storylines.