For me it is either Better Cities or Open Cities Reborn that is a "must have." I love them both.
Better Cities makes the cities look much grander, like real cities, not just towns.
Open Cities basically just opens the towns, so that they aren't, and this don't feel like, different worldspaces.
Open Cities Reborn goes one further; opens the towns and rearranges them, sometimes significantly. If you are familiar with the original layouts; then the effect is spectacular (especially now they include differing wall types and so forth.)
It is also worth noting that Better Cities now as an "open city" option for its cities as well. But I have yet to check that out.
I was a "Die Hard" Better Cities user, but recently switched to Open Cities Reborn, and am loving it. I was missing some of the cool stuff from the Better Imperial City (Open Cities doesn't touch the IC) but much of that has been mitigated by adding "ImpeREAL City Unique Districts" to my load order. Which adds some of what Better Cities did for the IC.
I'm loving that combination right now. But can't really recommend one over the other: Better Cities or Open Cities Reborn; I recommend taking whichever takes your fancy.
I have corrected Vargr's post, above. Vargr named the wrong mod when talking about opening and rearranging the towns.
Better Cities' Open Better Cities option is the same as Open Cities, except that it includes all the content added by Better Cities.
You can use Open Cities Reborn, and still use the Imperial City part of Better Cities as well (don't use the Open Cities IC Outer Districts ESPs).
If you're talking about IC expanded - I'd stay far away from that mod. Read the comments.
If you're talking only about the IC tunnel mod - BC has a better version I think.
The Better Cities archive includes a cleaned copy of IC Expanded tweaked to be compatible with Better Cities. It should not suffer from the CTD issue of the original mod, as so far no one has reported still getting crashes when using the BC-tweaked version.
The IC Tunnel used in Better Cities is integrated from a separate mod which was never finished, however that mod is no longer available. The version in BC has been enhanced, NPC AI packages completely redone, and the tunnel merchants now have their own shop alcoves to sell their goods in. It still needs more work though.
There is an ESP provided on the Better Cities Nexus download page, which is still work-in-progress, adding most of the Better Cities buildings into Open Cities Reborn, so that players can enjoy the best of both mods. Not all cities have been done yet however, which is why it is still WIP/beta.