There have been a lot of debates on who will be Luffy's equivalent of Garp between Smoker and Koby. A lot of fans were shifting toward the latter because of Oda's mistake of putting him at Rear Admiral. The fact is that he is still a Captain and there is no way at that level you can be a threat to a Schichibukai and even less to a Yonku. Make no mistake, Luffy's friend will become an Admiral by the epilogue of the series but it seems too early in the present.
Imho, Smoker is closer to be the Garp of this generation for obvious reason than fans have already listed:
-Swore to pursue Roger/Luffy to the end of the world
-Almost killed/captured them many times
-Had to team up at some point (God Island/Punk Hazard)
-Acknowledges their opponent and respect them although you wouldn't call them "friend" (Luffy telling Smoker he wants to fight him at his best at PH, Smoker leaving the SHs alone after Luffy saves his life at Alabasta)
-Garp doesn't want to talk about God Island because he teamed up with Roger a pirate, Smoker told the WG to eat shit when they gave him credit for bringing down Crocodile when it was Luffy's doing.
- Smoker even has a scar across his face like Garp which is reminiscent of Zoro and Rayleigh eye injuries.
When I read chapter 957 and Sengoku said Garp allied with a Pirate. I knew it had to be Roger if Luffy and Smoker mirrored their relationship and I was right. Even the story about covering up Roger's involvement and giving the credit to Garp is identical.