Inca are probably my favourite civ and i would argue there one of the best, why?
-free/half-price roads which ease economy worries in the early game and allows to build up more units without worrying about debt. Generates quite a significant amount more with a large puppet empire than usual
-no hill penalty, very useful for quick deployment of any unit and withdrawals/swaps/setting up artillery if needed. the greatest thing i like about this is that it applies to every single unit. There is hills essentially everywhere which makes this ability useful practically anywhere on the land-map, throughout every age in the game
-slinger withdrawal bonus and upgrading a whole bunch of slingers to crossbowmen to carry on that attribute (very useful for creating traps for AI units to fall into - even though they seem to do that well enough on their own anyway)
-Good at settling in harsh locations thanks to the terrace farm, opens up many locations for them.
-Mountain biased sometimes offers great protection/isolation from other AI's that will covet your lands.
For me, there definitely a top-tier civ. i do have to agree with an above poster though - there is nothing more aggravating then finding a hill surrounded by 4 mountains, only to be settled by a sheep