I too think he's becoming what Garp was to Roger and for now he has been a wonderful rival to Luffy. They have a very different personalities, yet they also share something. They have this strange cat and mouse game which isn't just about hating each other. Luffy has straight-out told Smoker he doesn't hate the marine and Smoker, although having a some sort of grudge, isn't totally blinded by his obsession to capture Luffy.
Also, Smoker is one of those marines who isn't mindlessly following their justice but instead does what he thinks is right. He doesn't just obey every command like a loyal dog. He actually sometimes acts downright rebellious if he doesn't agree with those "old geezers" in the government. He obviously has a sense of honor since he returned the favor when Luffy saved his life and he also refused to take credit for something he didn't do a.ka. defeating Crocodile (though in the end the WG didn't care about his opinion). He also didn't chastise Tashigi when she refused to capture the Strawhats even though she had a perfect chance. BTW I think Tashigi makes a great subordinate for him and that interaction is one of the reasons why I like Smoker.
I also really like Aokiji, Garp, Kizaru and Akainu. Oh and honorable mention for Bogart (Garp's right hand man) even though his screentime is almost non-existent. The guy just fascinates me. I hope Oda hasn't forgotten him =P. Coby and Helmeppo aren't bad either and I'm interested of their development in the future. Fullbody and Jango instead are awesome pair for the lols.
Also… another honorable mention for the filler marines Jonathan and Drake (from G8 arc). I really liked those guys. Drake was hilarious while Jonathan was very likable with his lax attitude and quick wits. I REALLY hope they would be featured in some upcoming filler again.
Whoa. I kinda like MANY of the marine characters. Though Smoker is my favorite. Aokiji would take the second place I guess but after that I can't really say the order.