I've unlocked Hexagon, Hexagoner, Hexagonest, Hexagon (Hyper), Hexagoner (Hyper) but not Hexagonest (Hyper) so is there any hidden endless level to be unlocked after I complete them all or Super Hexagon doesn't have any endless mode?
There are only 6 distinct levels in Super Hexagon (those that you've listed.) There is no hidden endless mode beyond this.
The sixth level, Hyper Hexagonest, has an implicit end - once you beat 60 seconds, you'll get to see something nice. All other levels are effectively endless already - i.e. you can theoretically continue playing until you hit a wall (either within the game, or in the real world out of frustration.)