Boston Celtics guard Marcus Smart is one of the best defenders in the NBA. The rugged defender doesn't back down from anyone and plays with a grit that Beantown fans love to watch on a nightly basis.

However, there are a few players in the NBA that even Smart admits are nearly impossible to defend, even for him.

In a piece for The Players' Tribune, Smart lists Kawhi Leonard, Paul George, Kyrie Irving, Stephen Curry and Anthony Davis/Giannis Antetokounmpo as the toughest players to guard in the league.

For Smart, Kawhi is the “most unstoppable player in the league right now.” He cited The Klaw's ability to score the ball in different ways at his size as a big hurdle for a defender like him.

I would definitely say he’s the most unstoppable player in the league right now. The way he can score the ball at his size — he can take you off the dribble, post you up, shoot the mid-range or shoot the three — it’s wild.

Kawhi is just … different.

He’s out of this world.

As for George, Smart is impressed by how he can “stay hot all night.” The Celtics guard also described PG's game as “smooth”

Paul’s game is all about rhythm. It’s like when guys see a shot fall once, then twice … that momentum builds. It’s real. Being in the zone is real. The hot hand, being on fire — it’s all real.

On the other hand, Smart sees Kyrie's ability to “ad-lib and come up with things on the fly” as a special skill that is difficult to prepare for. In fact, he even said that he calls Kyrie's moves as “last-minute moves.”

All you can really do is stay up on him. You can’t fall for all the shakes. He’s gonna throw a lot of moves at you and pick his spots. So you gotta stay on your toes. Stay disciplined.

And be ready for anything.

While Curry has already been known as an elite shooter that is difficult to stop, Smart didn't hold back in saying that there's no bad shot when it comes to Steph. Sure, Curry can have an off night, but a defender like Smart can never take a chance on the Golden State Warriors sharphsooter having a bad night.

He can hit from anywhere. He’s liable to shoot from half-court and hit it. And he doesn’t need a lot of space to get his shot off. He’s got a quick trigger. Even when you’re right up on him, that little inch of space you leave because you think you’re close enough?

You’re not.

Smart then highlighted Davis' raw strength that makes him really unstoppable. The 25-year-old even recalled a story to prove his point about AD's dominance, especially in the painted area.

I remember one of the first times I guarded A.D. He got me down in the post, but I had great position. I was using my strength to keep him away from the basket and I was forcing him to where I wanted him to go. I was feeling good.

And then he literally just turned and shot over me.

Like it was nothing.

For Giannis, Smart acknowledged that the Greek Freak's physical tools and the ways he uses it makes him a matchup nightmare. Smart specifically pointed at Antetokounmpo's wingspan  and speed as his key strengths.

But Giannis different. He’s out of this world. Yes, he can handle the ball like a guard. So can A.D. But with his wingspan, the way he can manipulate the ball and get you chasing it one way before coming back with it — because he’s also quick like a guard, so he can cross you if he catches you leaning … man, it’s special. Plus, he’s also ridiculously long and athletic. So where it takes most people four or five steps to get to the basket from the three-point line, it only takes Giannis like three. Plus he can jump out of the gym. Plus he’s strong enough to get through contact, so you can’t body him.

It's worth noting though that players like LeBron James and Kevin Durant are not on Smart's list. After all, the two are frequently called as the best players of this generation. However, Smart certainly has every right on his own list.