There is no evidence that Roger and Garp were equals. However the fact accomplished what Shiki could not (cornering Roger , not just once. But many times). This feat is nearly impossible for a buster call , this feat is impossible for an admiral with 20 battle ships (would Shirohige be cornered by 20 battleships? LOL NOPE).In no particular order:
Dragon
Akainu
Aokiji
Shanks
Kizaru
Mihawk
Garp
Sen Goku
Kaidou
Blackbeard
Sorry, but what is this nonsense? As far as I know, we have no proof when exactly Garp was equal with Roger. What we do know is that Garp was in fact not as strong as Roger when Roger was at his strongest, because only one man was as strong as Roger at that point(stated by Buggy). When Blackbeard got Whitebeard's devil fruit, he managed to do what Whitebeard was doing, other than having control over it, and that was only done by studying the fruit for years. Two years later, when Jinbei was talking about the two new changes the strawhats needed to know, he mentions Akainu and Blackbeard. We find out that Akainu became the new fleet admiral, and that Blackbeard became a yonko. He was not referred as the strongest man now that he's had the fruit for two years, but as a yonko. So yes, Whitebeard being the strongest man was not because he had the strongest fruit, but because he was the strongest man in the world. What's next, Mihawk is the strongest swordsman because he has the strongest sword? =S.
Roger did genuinely state that Garp had brought Roger near death many times ; which is why he trusted him the most to bring up Ace.
Garp>Admirals
The Yonkou>Admirals
It's safe to Say Garp is minimally as strong as the weakest Yonkou (currently Teach)....
Mihawk is the strongest swordsman due to his skill and Haki.