lots of hot air to follow
I have played all mainline Civs and Civ Rev 1.
Played 1 and 3 the least, more due to life situations rather than (my perception of) game quality.
Civ 2 is my nostalgic favorite - In college, during the school year, friends would huddle around one computer in a dorm room, lots of pizza and, errr, soda

and play by committee (guy in the drivers seat got final say) - we also did this with Masters of Magic around that time, a game that I think I'd like to see remade, but would probably ultimately disappoint - but I digress. Over the summer, my buddy worked the 3rd shift and he had a better PC than I at the time, and a copy of Civ 2, so I would drop by his apartment just before he left for work and would play Civ 2 until he got back in the morning. No, I didn't attend many classes over the summer, why do you ask?
Civ 4 - I can understand logically why others find 4 great / the best, but for whatever reasons it has the most negative or ambivalent memories of all the Civ games. I probably put way more hours into 4 than 2 (again due to life circumstances, 4 came out when I was single, lived on my own, and I had plenty of time to myself in evenings to invest). I have enjoyed reading and watching other people's play-throughs more than I have playing it for myself. I've tried to figure out what it is about 4 that just did not capture me ( I say knowing I put in triple digit hours

). I think there is a micro/macro management balance where maybe it was too micro on Civ/City aspects, which seemed out of whack for the macro management of just tossing your units into one big stack. I am also certain that multiple people here could tell me how wrong I am about this.
Civ 5 - again, I can understand reasonably why it supposedly isn't as good as 4 or other Civs, but I have enjoyed playing 5 more than 4. I most dislike that its a performance hog. I don't ever recall any other strategy games feeling as slow as Civ 5 has / does - and generally my PCs are above-average in respect to other gaming PCs of their time. In contrast to 4, 1UPT tipped the combat into being more micro than the Civ/City aspects which seemed too macro. that is also not my ideal but I think it "played" a bit better for my tastes.
My hopes and impressions for 6 (hopes are unlikely I know):
- excellent performance which allows for larger maps. 5 crashed or chugged along way too often on large maps and even today I see plenty of visual glitches that pop up mid game, you get the feeling that the PC is struggling to keep up with the map. This shouldn't happen in a strategy game - tone down the graphics / animations.
- Related to the above - I want the game to perform and have a UI that can scale well to 4k resolution. I realize I'm asking alot here.
- Better balance between micro and macro management of all aspects of civ - Your Civ as a whole / Cities / Combat.
- xUPT - I have seen that there is some semblance of an "Army" concept in 6 which is moving in the right direction, but I am skeptical that it will be exactly to my tastes - I think the sweet spot - given the typical Civ Hex/Sqaure "size" is 4 or 5 "full strength" units as opposed to 1 unit with a couple of "support only" units.
- Unit Types - I would like to more depth to leadership units (e.g. "normal", not "great" generals/admirals). As well as to Scout (Recon/Stealth). Leaders obviously would be more "support" unit
- I look forward to the city un-stacking, I think it has potential to be the one feature that will have universal appeal and sticks with Civ series onward. ( And I'll try to edit this out if its a disaster

)
- The leader specific traits separate from civ-specific traits tease/imply that there will be multiple leaders per civ. This is something I would like to see more of in Civ without needing an expansion - unfortunately the increasingly extravagant productions for the leader screens means that this is unlikely to ever happen, and I fear that this potential feature is simply a convenience to the modding community.
Last gust of hot air:
There has never been a perfect Civ, even with the rose-colored glasses of nostalgia, and I don't think there ever will be. But they have all been enjoyable in their own ways and I could not imagine purposely "skipping" a Civ release unless I was unemployed and broke, it is a gaming staple and the dollars to hours ratio means I get quality entertainment for pennies per day. On the whole, I believe the quality has trended upwards and see no reason to expect otherwise with 6.
So, I will be buying Civ 6, and barring intervention of the wife I will probably put up triple digit hours into it within the first couple of months, get distracted by life, and then notice the sale on the "complete edition" on steam, purchase and then sink another few hundred hours into it (on a positive note regarding potential wife obstacles, I recently got back into 5 - bought G&K and BNW - and have re-introduced the wife to Civ - she also has memories of playing a lot of original Civ and Civ 2 - while teasing details of the upcoming 6, so fingers crossed there, she just started her own game with Elizabeth last night

)
It is highly likely I will pre-order the "deluxe" (although I generally "pre-order" one day before release - part ensuring release date is correct, partly not wanting to give a game company free interest, and part because I'm a procrastinator - OK one of those weighs more heavily than the others

)