While I greatly enjoy D&D 5th Edition, it isn't without its issues. One of those is the Beast Master Archetype for Rangers. My players have some major issues with it and what what I've seen my group isn't the only one. I will say right off the bat that if you like the Beast Master as it is there's nothing wrong with that and keep using it as written.
What's Wrong With It:
The shortest answer is that a ranger has to sacrifice their attack for the beast to attack. From an "action Economy" standpoint I get it as letting a players attack twice in a turn isn't meant t be allowed untill 5th level and only then for martial characters, but wait a second., First the Ranger is a martial character and gets his extra attack at 5th level (this does ease up SOME of the burden letting the beast and hero attack, but it's not great). Second plenty of characters can attack twice per turn by this time with the use of a bonus action. Anyone who dual wields can do this and the fighting style for it makes that second attack jsut as effective as the first. Even the Berserker Barbarian (Another class with issues for totally different reasons) can enter a special rage where he can make an extra attack as a bonus action even with big two handed weapons. Third, and possibly most importantly, thanks to hirelings a player could just buy a dog and while it doesn't have the bonuses of the ranger's beast it can attack all on it's own.
I'm sorry but just having a pet should not be more effective than a class ability. Sure with mount rules if you happen to be riding an animal (with limited Intelligence) you have to use your action to let it attack, but that's because it's a mount and STILL that's any old animal you can just go buy. A freaking horse can do this just without the bonus.
How to Fix It:
The obvious answer would be to just give the beast its own actions and be done with it, but honestly that is a bit over powered when you factor in the bonuses. Since your proficiency bonus is added to the existing bonuses an animal's bonuses meaning the proper combo can make for a VERY effective killer to the point that the ranger isn't even needed in combat.
The fix I came up with tweaks some of the mechanics of the Ranger's Companion Companion and breaks some of it up into different abilities (though you still get them all at level 3)
Choose a beast that is no larger than Large (talk about that later) and that has a challenge rating of 1/4 of lower. Add your proficiency bonus to the creature's AC as well as any saving throws and skills it is proficient in. It's hit point maximum equals its normal maximum or four times your ranger level, which ever is higher.
The beast obeys your commands as best it can. It takes turns on your initiative, though it is limited on the actions it can take without being commanded. On your turn you can verbally command the beast where to move (no action required by you, however if commands are not given the beast my act independently). You must use a bonus action if you wish to command your beast to take the Dash, Disengage, Dodge, or Help actions.
(Notice the bonus to attack and damage is removed. That's because it comes in with a different ability. The Attack action is removed and the actions revolving commands are also modified to allow a beast to attack or continue attacking an obvious threat without the need of commands however this is at only the effectiveness of the beast natural abilities.)
As a bonus action the ranger can designate a target for their beast to focus on. Until told to stop the beast will continue attacking even if the target may already be dead.
(That takes care of the attack action and even adds a bit of "set and forget" but limited to the animal intellect that would drive a normal animal to continue doing as its master asked or even jsut pure blood lust in the case of some more violent animals.)
As an action the ranger can point out points of attack for their companion adding their proficiency bonus to the beast's attack and damage rolls until the beginning of your next turn. One you have the Extra Attack feature this can be used in place of one of your attacks.
(This still limits the effectiveness of these more precises and powerful attacks more or less giving the ranger their original attack option while providing weaker alternatives.)
A Side Note:
I said further up that I would explain the upgrading of the beast's size to Large. This is because limiting it to Medium is simply unfair to player choice. I feel it was implemented to disallow players from using their beast as a mount, however Small sized players still can do this. Constrictor Snake, & Riding Horse are the only beast from the PHB that it even adds and adding the Monster Manual only ups that to, Axe Beak, Camel, Draft Horse, Elk, Giant Bat, & Giant Lizard. Yes some of these are awesome choices, but if you're thinking flight from the giant bat of the climbing ability of the giant lizard are game breaking remember that a small character can take a vulture or giant wolf spider already. I would say some of these animals are limited by the area and maybe even favored terrain of the ranger, but that is GM discretion. Anyone can buy a horse so why can't the ranger who can call an animal to his aid be able to call in a horse or elk. these are freaking tropes of the genre and yet someone decided that large beast breaks the game.
Anyways like I said this is my fix for Beast Master Rangers. If you like it feel free to use it. If you don't that's fine too. I will also point out before I go though that if you're allowing multi-classing then you might actually want to keep the older version since the beast bonuses are tied to proficiency bonuses and not class level this option mixes well in multi classing with minimal sacrifice.