Run on the track like...

[Jome]

Jesse Owens.. Nah never that
I mean, when a rapper starts out, he's usually less skilled then he will be later in his career. But to be a good rapper, you must "Run on the track", or rather sound like you are the master of the beat; you OWN the beat. This is often achieved if the rapper makes the beat himself, so that he can fit the beat to his style. To run on the track, you must have a strong voice/strong personality or just plain charisma.
Earlier examples on this is: Pac, Biggie, Eazy, Cube, Slick Rick (still)
Today: MC Ren, Method Man, Xzibit, Nas, Jigga, Eminem, Noreaga, M.O.P., Kurupt (Sometimes), Snoop (Sometimes), Redman, Tray Deee, Jayo Felony, Big Boi, Mystikal, Pharoahe Monch, Styles, Ghostface, maybe more.
Runner-up: Crooked I. I don't think he got IT yet, but he's steady improving.

[Trauma-san]

Interesting aspect.  

I'd say Biggie and Eminem "run on the track" the best.  Biggie was good about pausin' for a second, lettin' the beat catch up, then just do a Little, "Um, yeah, this is for tha honies, makin' like they know me, you know they want that money" or whatever (I made that up), but you know, just feelin the beat, rhymin' with the beat, goin' up and down with the beat, staccato flow, slow like, just killin it.  Perfect Example... listen to "What's Beef"... "I done smoked-wit tha best of um,.... ah, shot at tha- REST of um... was about a hundred a mor ...  - maybe less of um"  Just the way he slow flowed it, was perfect, perfect with the beat, never be duplicated.

Eminem... I'm always reminded of his KILLER, maybe his best ever (probably), and MAYBE one of the best verses by anyone ever, on "Dead Wrong"... had that taca taca taca beat, you know, kinda snappy with drum sticks, weird dancin' beat... and he busts in "There's seven diff'nt levels a devil worshippin' horses heads"   RIGHT on top of the beat.  Sounded for a minute like he was the beat. "Who's ready for war? I'm ready for war, I got, machette's and sword's for any faggot that says that he's sore, My uzi's as, heavy as yours, Yeah, you met me before, I just din't have this large an arsenal of weapons before" Perfectly matched the beat, runnin on it.

Dr. Dre did this the best on "Let me ride"  Listen close to how he did the drums... the first verse, they're kinda sparse, just a drum beat... last verse, he's double timin' the drums, and he breaks out with that incredible flow, RIGHT ON the drums, perfectly mimicking the drums with his flow... "So on, and So on, won't ya let me roll on? I rememba back n' a day, when I use ta have ta get my stroll on".... Listen to it, you'll hear the drums and his voice turn into one instrument on that line...

I'd say Kurupt, wayyyy before snoop, like on ain't no fun... "Well, if Kurupt gave a f*k about a bitch, I'd always be broke... I'd neva have no motha-funkin' indo ta smoke, I gets, LOCED and looney, Bitch, you can't do me, do we like BBD, you hoochie-groupie....."

[Jome]

Hmm. good observations there. Em is one of the best at this today, but as I heard Slick Rick on that De La-collabo I was fairly impressed. He sounded so smooth that you think he wrote it /produced it & mixed it, with his voice just matched in with the beat. "What we do for love" it's called, and it's
MC Ren & Method Man also got this kinda flow now, that you'd think the beats was made FOR them. MC Ren betta release a solo-album soon, with all those hot guest-spots he puts in.

[Jome]

Bump

[PLANT]

listen to gza on "breaka breaka"
he runs that shit...
the name rings a bell, killable, w/ two sylables, the wu was commin thru, the outcome is crytical...
he spits perfect w/ that beat right there...and snoop on tha shiznit is amazing
"set trip, gotta get em fo his grip, as i dipp around a cona, now im on a, notha mission, wishin, upon a star, snoop doggy dogg w/ the caviar, in the back of the limo, wit demo, this is the real, breakin niggaz down, like evander holyfield
thats one of the best verses snoop ever made and flowed on, he had a mad bite in that track....wish he still had that tone in his voice

[DreSnoop00]

damn I say everyone needs to get their best flows back

[Big BpG]

give Snoop a laid back beat and some time and you got yourself one of the best rappers in the game...

"All the kids used to rap as fast as they could, but me, i took my time... thought while i rapped and got respect for what i did" -Snoop  about his days of freestyling

-Big BpG