Well, I personally think that the matchmaking/highest skill in Halo 3 is actually broken, courtesy of all the boosters/2nd accounts that now seem to infect pretty much every playlist. So with this in mind, I would give yourself some very realistic targets if you're looking to improve your highest skill, as you'll reach a certain level and get stuck, as you can't string enough wins together to keep increasing, as you'll come up against the team of 5 star generals who are all playing on their 2nd accounts......in Mexico......and they get host.
Having said all that, I think there are playlists that you could move up in, and there are some definite ones to avoid.
SWAT is worth a try, as once you get to something like level 10 you should start to play more genuine people (you'll get boosters at 0-10), and assuming you do quite well in the playlist, you could get up to 20-30 pretty quickly. Another good thing about SWAT is that the games are very quick, so you can rattle through a few games in quick succession, and if they're winning games, then you could climb a few ranks in a short amount of time. The drawback with SWAT is that it ultimately comes down to who has host, as that split second advantage can make all the difference....
Another (if slightly more drawn out) good playlist is Team Slayer....again, I find there are slightly less boosters in this playlist, and seeing as it will still take a well placed grenade and a few shots from the BR, you've got a chance to escape and regenerate your health. Your best bet would be to add some of the guys who frequent the Halo threads on here, and jump in for some Team Slayer action.
And quickly, the ones to avoid:
Team Doubles (booster/2nd account central)
Squad Battle (should be renamed Lag Battle, rarely get playable connection)
Lone Wolves (takes too long in my opinion)
Good luck on your quest. I personally have given up all hope of improving on my 46 (in SWAT), as I just think that the system has been broken by so many idiots running boosting accounts. But I do think its still perfectly feasible to get to 30-35 fairly easily.