Venice is clear #1 most different, but lots of othrers are very different. Shoshone
1) Build city as with each other civ
2) Wow more tiles
3) Get ruin as with each other civ
4) Wow I can choose bonus
5) Almost verything else is usual
Nice and useful stuff but I wouldn't really call it such revolutionary
1) Ally city states
2) ...
1) Make Academy from this scientist
2) Scientific Victory like with each other civ
IMHO The Most Unique Civs:
Maya - early Pyramid spam + this ingenious and interesting ability
Inca - completely altered city development
Venice - obvious reasons
Zulu - with Zulu you do exactly opposite stuff to Usual Strategy: spam melee infantry and kill everyone regardless of everything
Mongolia - same thing with Keshiks
Arabia - less militaristic but Camel Archers allow on unique warfare + double range and pressure caravans offer interesting choices + more copies of all luxuries via Bazaar!
Huns - even more extreme than Zulu: you should literally do nothing but kill everyone since first 0

(and it is lot of fun)
Indonesia - people say it is weak but personally I waaay prefer all these unique bonuses to extremely boring Babylon one - trick pony (writing -> academy -> science lol)
Austria - I don't know why it is displiked, it has its own unique interesting feature.
Sweden - completely alters diplomacy
Poland - free policies stuff is simple but offers much depth in strategic choices, also Winged Hussars offer completely unique type of military domination
Assyria - who cares about science, let's conquer cities
Polynesia - thalassocracy
Persia - you actually have to care about your Golden Ages and timing
Aztec - Trait and Jaguars completely change early game. Many interesting strategic choices.
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Most boring, passive civs:
- Ethiopia, you build faith monuments, get easy early religion, sit and defend with boring defensive bonus and unit
- China, you do exactly the same things as with every civ - just better
- Babylon -> free science, that's all
- Greece -> yeah it's so fascinating and engaging, I lose CS influence slower. Yawn.
- Russia -> more numbers
- America -> fast scouting, cheap tiles.