I think Halo 3 has a shot, but I feel pretty confident that Galaxy takes the cake in the long run. I'm thinking more along the lines of how long will it take Galaxy to overcome Halo 3's sales.
I think Galaxy might reach 2-3 million in Japan. It's certainly possible it could be more with the legs that Nintendo titles have been showing in Japan(NSMB, Brain games, Wii Sports/Play). But I honestly didn't think that Zelda would sell as poorly on Wii as it has in Japan. In the long run TP may match WW's sales there, but with the way the Wii is taking off there it should be selling better. You could say Zelda is not the blockbuster it once was in Japan, but it looks like there is plenty of interest in PH for DS. This brings me to Sunshine, which sold only marginally better than WW in Japan. That's why I think that Galaxy won't necessarily be huge on the level that NSMB is. I'm making a safe prediction of 1.5-2 million in Japan for Galaxy's lifetime sales.
I really think that Galaxy will be enormous in America, and to a lesser extent in Europe. Lifetime sales for it could reach Halo 2 levels in America. Sunshine sold pretty well here on a system that just never really took off, so I can imagine what it's going to do on a hot seller like the Wii. Also consider that a complete turd like Mario Party 8 sold over 300k in just a few days in May. Twilight Princess(Wii/GC) has pretty much already caught up to WW's American sales in less than one year and is showing legs that WW never had at the $50 price. The one thing that Galaxy may have going against it is releasing so close to Brawl, but that's more of a getting started problem, not a long term one.
Speaking of Brawl, I think it's going to end up being the Wii's best seller over it's lifetime unless you count the massive pack-in numbers of Wii Sports. SSBM had legs like no other GC game in America and Japan, and was the best seller for each. I also think that SSBM was the only GC million seller in Japan, and I don't see those numbers getting smaller with it being on the Wii. There's always the chance that it could suffer the TP paradox(not selling as well as it's predecessor on the less sucessful console), but I think Smash has great casual appeal in Japan, moreso than Zelda, and maybe even more than Mario in general(not counting NSMB).
Halo 3 is a tougher one to nail down for me. I really don't see it selling less than 2 when I first consider it, since the 360 looks to be the solid number 1 or 2 console on America(depending on how you view things), but I think most Halo 2 owners have a 360 or will be getting one this holiday season at the latest. I really don't see it pushing the system like the first Halo did the X Box. In my mind, Halo 1 sold X Box's. The obsession with Halo turned a lot of people from "bought it for Halo" fans, to straight up X Box fans. So unless MS can grow their userbase through other means, Halo 3 may have a glass ceiling around Halo 2's numbers if the 360 can only moderately outsell the original X Box. The thrashing that the 360 is giving the PS3 makes me want to say that Halo 3 will outsell Halo 2 by a decent amount, 1mil+, but the console sales seem to be running out of steam in America and Europe. A price cut will really help, but if Sony doesn't drop their price considerabley, I don't see MS dropping the 360 to $200 in enough time to make a huge difference. Considering that MS almost always loses money on the X Box division, and the fact that the faulty 360's are costing them another billion and then some, their hands may be tied for some time before they can get to $200(which will be a great price for the 360 IMO). I'm also beginning to think that maybe the "sweetspot" price this gen may be $250 for the higher end consoles(360 and PS3), instead of the traditional $200 spot. MS may not need to go as low as $200, maybe $250 is where they need to get by next holiday season. They obviously need to get to at least $300 in the meantime, or their console won't sell enough in America in non-holiday seasons to attain a strong position(against the Wii), or a dominant position(long term against the PS3).
One thing that could really throw all of this out of wack is the fact that the 360 may well be the number one console this holiday season in America and possibly Europe. If Nintendo can't meet demand in non-holiday seasons for the Wii in any territory, it won't have the millions and millions it could sell during the holidays. They have to get production way up in time for the holidays, or to be stockpiling large amounts to prepare for the holidays. If neither of these things happen for Nintendo, the 360 will be the best selling console this holiday season(at least in America and Europe).
Anyways, Galaxy outsells Halo 3 in the long run, but not out of the gate. If I have to give numbers, I'd say Halo 3 worldwide sales during the lifetime of the 360 around 9-10 million. Galaxy worldwide sales during the lifetime of the Wii around...11-13 million.
"I feel like I could take on the whole Empire myself."
