Unlike consoles, computers do not come prepackaged with gamepads. This is leads to the fact that also unlike consoles, there is no "standard" in gamepads designed for the computer. Since manufacturers won't know what to expect the player to have in terms of interface, it's largely up to the consumer to determine what gamepad works best for them. This is why, unless they've drastically changed things in modern times, the keys can be remapped as you see fit. This lets you have greater flexibility in what device you use for game, regardless of what console it was originally on.
At minimum, to play the games that are designed for the current generation of consoles, the most important thing is to have enough buttons. 4 sets of shoulder buttons, 4 face buttons, and a set of start/select should be sufficient. You may consider investing in a gamepad that has multiple joysticks and directional pads if the games you need will require them.
Once you get past the part of having enough inputs, then the remaining thing to do is get what feels most comfortable for you, or what you will play best with.