Best? GTAIV. Most enjoyable? Crackdown or Saints Row. Funnest? Dead Rising.

You people saying crackdown isn't number 1 are insane. Playing Crackdown with the bonus features active and all the cheats active (Super Strength and Agility, literally rank 5 each) makes the game so very very very fun to play. On top of that taking the armored car and launching rockets down the street traveling at ridiculous speeds is way more fun than anything I've done on any grand theft auto game.

Plus the Mini-gun Buggy is fun too.

Crackdown for me

I've played the four you mentioned in the original post. Dead Rising only the demo though.

Saints Row gets my vote for first
Crackdown
GTA IV unfortunately I haven't done much in it so I haven't had the whole experience with it.

Grand Theft Auto IV, then Saints Row.

1. Crackdown
2. Saints Row
3. GTAIV

Crackdown only beats out Saints Row because of the co-op mode, plus the orbs are addicting.

Of the ones you listed, I made it all the way through Crackdown, Dead Rising and Saints Row. I played Crackdown and Dead Rising again, starting from fresh gave saves (i.e. erased prior ones) and played them all the way through.

GTA IV? I made it about 10-12 hours, was fairly bored, and never had the urge to play the game again. Just being honest.

Crackdown
mercs2
deadrising
GTAIV

havent played saints row so cant judge

Depends on what you consider "best".  All the other other three are more enjoyable or more fun for you - then why is GTA the "best"?  Just curious.  If I am bored out of my mind, I don't care how good the game is "supposed" to be.  It ain't great.   To me, that was the great failure of GTA IV, after all the hype and the glitzy cover faded away, you were left with a game that just wasn't all that fun much of the time.

I haven't played Dead Rising.
But, out of the three I played, I'd go with Crackdown.
It's not really Story-driven or cinematic experience.
Just little menus pop up and tell you to murk a boss, but the actual gameplay is fun.

Saint's Row is also pretty good.
Try to pick up both if you have a chance.