Need For Speed has existed for 26 years and there are lots of games in the series on consoles and portables. So picking a top 3 will be difficult because nobody would have played them all. Pick your favourites in the series whatever you've played of it.

Need for speed underground 2
Need for speed most Wanted
Need for speed 4

Underground best sold need for speed game series with each 10m sales.

If they just made an underground 3 instead of the halfbaked reboot.

Most Wanted sold 16 million.

Hot Pursuit, Most Wanted

Pretty much THIS. The PS3 versions!

Most Wanted (2005)
Underground
Shift

Played nearly all of them.

The Need for Speed (3DO)
Need for Speed Most Wanted
Need for Speed Underground

The worst - Need for Speed Undercover

I own many but have only spent considerable time with 3 so I'll list those. Ordered by how much I enjoyed them:

1. Underground 2
2. Underground Rivals (can't beat the nostalgia of a launch title!)
3. NFS 3 Hot Pursuit

This one is the best, no contest:

The rest are more or less trash, but 2 on PC holds a special place cos it was one of my first games in 1998, but I wouldn't be able to play it today.

Hot Pursuit 2 is my favorite too (though I played it on PC),  followed by NFS III. I didn't like that much when they went open world.

NFS 2 was also one of my first games, its soundtrack is so good that I still listen to it, one of my favorite songs: