I think he is
Discuss 
How do you compare him to guys like Walter Johnson, Cy Young, Christy Mathewson and Warren Spahn?
From 1960 on I am not even sure he is top 5. You got a ton of competition with guys like Koufax, Seaver, Maddux, Clemens, Pedro, Mariano, Gibson, etc.
Yes.
NO HE'S NOT
no argument places him in the top 5
arguments could be made that he isn't top 20 (i personally think he is some where in the 14-17 mark)
There are a lot of greats but what he did in the steroid Era blew my mind. And his name never was never in the steroid conversation
Hard to compare him with Cy Young era, but modern he trails Gibson, Koufax, Ryan, Clemens, Maddox off top of my head 
Spuds Buckley - Hard to compare him with Cy Young era, but modern he trails Gibson, Koufax, Ryan, Clemens, Maddox off top of my head
seaver
palmer
fernando
pedro
mariano
"The Big Unit" is top 5 all time nicknames for sure though
Birds hate him.
Top 5 dad all time IMO.
Nolan Ryan was waaaay better
no if you are looking at pure stats. the problem is that the stats are relative to the era. i have no doubt that the earlier eras did not have a lot of 100 mph pitcher who where southpaws though.
i have maddux as the best of all time.
if you are looking at pure ability (how good were his pitches, control, velocity consistency and intangibles),
then i'd rate him top 5.
have a 6'5 monster glare at you who sometimes threw 'wild' 100mph pitches. that was very intimidating.
i think guys like halladay, cone, stewart and jimmy key had much better control, but none of them threw close to 100. i'd still rate him below martinez and clemens though.