Randy Rhoads is the best guitar player that Ozzy has played with. Here's why:
When it comes to song writing, time has shown that Randy wins that contest, based on set lists alone. When you compare Randy, Jake, Zakk, Gus G, I forgo the "memorable solos" part because that's more relative. IMO, Randy wins that, but I can hum any Jake or Zakk solo as well. Instead, let's look at composition and what was going on in the background. Listen to Revelation (Mother Earth), but not during the obvious solo part, but the piano solo; Randy had a TON of things going on behind it that is just fantastic. The breakdown in I Don't Know is another example. That sort of thing is a common thread throughout those first two solo records. To his credit, Randy mixed a lot of those parts into one part that flowed (and changed, depending on the gig, based on the bootlegs) nicely. So much so, that you don't notice anything missing. When I think of the guys that came after, those things don't really happen. Killer of Giants is the only tune that comes to mind that had a bunch of different intertwined parts going, so refresh my memory if I'm overlooking something. As far as the recordings go, Randy tripled his solos, and it doesn't seem that anyone else has since then. Granted, all of those guys are certainly talented enough to do it, but didn't.
Behind him, I'd say that Jake would follow. I think that Jake had a lot against him in that he was only in the band because Randy died; yes, Randy was going to leave the band, but his passing obviously forced the issue. Jake did a killer job on Bark at the Moon; and while the Ultimate Sin has its standouts, it pales in comparison to Blizzard and Diary (which even benefitted from the songwriting talents of Bob Daisley, as did some of the Zakk era). Jake also wasn't super into the Ozzy style and had to change his style to that feel. Guys my age will remember interviews with Jake when he said that he'd write certain stuff for Ozzy and keep certain things for himself (what became Badlands). He was doing his best in Randy's house, as Zakk would later on..... :) Also, you'll remember that when The Ultimate Sin came out, Ozzy went hair metal; look at those videos. :) NOTE: I loved those records and I don't have a bias against them. While there were some great moments (and I love Jake), time has shown that the Jake era hasn't held up as well as his predecessor. Jake, a great player, was a victim of timing.
And then we have Zakk Wylde; Zakk cut his teeth on Randy and Jake, was considered a Randy lookalike, and was made to get a graphic on his Les Paul (painted by Peter "Max" Baranet) to try to differentiate himself. No Rest For the Wicked was a solid album; it was a return to the "heavier" side, and when Miracle Man came out, I sure as hell paid attention. Great guitar sound, good look, GREAT solo. That record, to me, was Ozzy's last good record as an actual artist; once No More Tears came out, Ozzy became a corporation, much like Aerosmith.
No Rest for the Wicked was the last Ozzy album I bought because it was what seemed to be the end of his creativity; No More Tears is Ozzy's "Permanent Vacation" (another Aerosmith reference); yes, there are some decent songs on there, but the fire was gone and formula took over. Like Aerosmith, I looked forward to each new release, only to be disappointed by the single. For those that saw "Don't Blame Me", think of the part when they're working on the No More Tears song; a bunch of people working on lyrics and the album title. (Actually, I take it back; I bought Down To Earth; I thought that Gets Me Through was promising; enter the "downtuned" Ozzy era. :) )
Then we have Gus G; I honestly can't really say much about this era; I saw the video for Let Me Hear You Scream and my overall feeling was "meh." I believe that record was written with Zakk, but Gus played on it, right? Gus, to me, is a victim of timing like Jake was, but for different reasons; he's playing in Ozzy, Inc (nod to the Ultimate Sin video); anyone could be playing the solos on this, as long as the chops are there. I'm sure he's a fine player, though.
And then, the final part.....playing live.
People say that Zakk plays Randy's stuff note for note live. Nope. It's not. Not even close. It's also missing the FEEL. Joe Holmes actually COULD capture Randy's feel with his material, but obviously, that didn't work out. That isn't meant as a shot to Zakk; the guy is a great player of his own material. I LOVE his solo on No More Tears, Miracle Man, and some others, but my god, he has no business playing Randy's stuff live.
I've heard Gus play Randy's stuff live, and it's obvious that his heart isn't in it and he wasn't influenced by Randy (much like how Randy wasn't influenced by Tony Iommi; he hated playing the Sabbath stuff, but I'll get to that in a sec). It's understandable because he's young, and probably grew up in the Zakk era.
Jake. Jake had something. There was a fire in his playing and his stage presence. Jake took Randy's material and made it his own, and did a good job with it. Much better than Zakk did. Randy had to play a couple of Sabbath songs live. He's on record saying that he hated playing those songs. The difference in what he did and what those came after him did (with the exception of Jake E. Lee) was that he made it his own, and it stood up on its own feet (personally, I'd say better, but that's me).
Randy's songwriting, composition, arranging, and what he did with his predecessor's material puts him as the overall best.
I'd also say that as far as Tony Iommi goes, yes, Ozzy wouldn't have had a career without him. But Randy was a more proficient player.