Armored Core: For Answer is probably the overall best entry in the franchise, and beyond that, a top contender for action-oriented mech games with a western release. It is almost a straight upgrade to Armored Core 4, and barring reference to the previous game and a minor design importer, one can start here. There is complete mech customization, fine tuning with a limited earned resource, an underlying conspiracy with branching paths, and an actually difficult hard mode. The average difficulty is however higher than AC4, and you will most likely have to vary your design at least once, unlike AC4 which mostly let you use whatever design you wanted without too much trouble. Of note is the increased number of parts from AC4, which was lacking, and all existing parts return, although the stats have been modified to varying degrees. Also, the game is designed that you will need a minimum of three playthroughs to experience everything in normal mode, though new game + is seamless and keeps your unlocks. I can only think of subjective flaws, namely if you prefer older Armored Core games, lack the hardware to play, the varying difficulty curve, and the one repetitive gimmick that is actually featured in the intro cinematic.
Bottom line, if you like action-style mech combat games and can play this game, you should really consider playing it. It's easy to get into and hard to master, and that includes the customization.