I can't play without mods
Same here. My current list:
InfoAddict (the single CiV mod I could not play without, period)
City Limits (shows the 3-ring workable area boundary around settlers and cities)
Experience from barb kills raised to level 4
No conquered allies (resets CS influence to 0 for civs that are wiped out)
Goody Huts no maps (removes barb maps and local vicinity maps from hut rewards list)
Reseed! and Rotate map start position (allows you to scope out your world and gives option to start in any of the civ starting locations. allows you to find a decent starting location without having to restart 15 games to do so)
Promotions- no InstaHeal (removed that completely disgusting AI cheat from the game)
Removing jungle gives city production (forests do and jungles don't?? i_don't_thiiiink_sooo)
Reveal Iron early (see it after researching mining, as it should be)
CS View City (shows you on the main CS interface who is actually protecting each CS)
Religion Spread (adds a map to the religion interface showing areas controlled by each religion)
Summary Luxuries (adds a very handy pop-down list to the top toolbar, with all the lux info you need right there)
Summary Specialists (actually a summary of which great people are queued up and how long till each will pop next, similar in form to the Luxuries list, on top toolbar).
Trade Opportunities (adds another screen to the diplomacy screens, shows quickly and easily who needs the luxes you have to sell or the ones you need to buy, if they have the money to afford them, other good stuff)
And a few other misc. ones that I use occasionally but not all the time, depending on mood.
Many of those are the work of Whoward69 (William Howard). His stuff is no longer available on Steam Workshop, but can be found in the modding section of this forum.