My fav klassic title is 2, majority of people here prefer Ultimate Mortal Kombat 3 but in my opinion MK2 was more funny and easy to newcomers. The stages, outfits, hud, sounds are so cool. I feel like they tried to make MK more "serious" with MK3 and maybe thats why it lost a bit of its charm.
I know that its not a klassic, but a honorable mention to MK Mythologies Sub-Zero. If you think that 3 is hard, this game will be hell, even more if you beat it on medium only to receive a message saying that you need to play in a higher difficulty to continue.
My tip in UMK3 is to spam Noob Saibot fog and beat people while they can't hurt you with his combo (low kick, low kick, low kick, low kick) or master Sheeva combo in MK3, also you can use Nightwolf "forward, forward and some attack (I don't remember the button" to shoulder ram people to their death.