I know someone did a thread similar to this a while back, but I'm going to expand it a bit. Vote Margins are dependent on three things:
1. Population
2. Turnout
3. % Margin
The best combinations of the above 3 factors will result in bigger vote margins, although population is really more powerful than anything else. I'll be listing the top 50 counties in the country for both Obama and Romney to see what some of the most powerful and important counties in the country are election wise. I will not be listing % or actual votes, just the amount of votes for x candidate - votes for y candidate. Let me know if you want me to go beyond 50.
Democrats:1. Los Angeles, CA: D+1,331,570 Votes
2. Cook, IL: D+992,995 Votes
3. Philadelphia, PA: D+492,339 Votes
4. Kings, NY: D+479,892 Votes
5. New York, NY: D+413,115 Votes
6. King, WA: D+392,304 Votes
7. Wayne, MI: D+382,032 Votes
8. Alameda, CA: D+361,502 Votes
9. Queens, NY: D+352,143 Votes
10. Prince George's, MD: D+312,204 Votes
11. Bronx, NY: D+309,244 Votes
12. Santa Clara, CA: D+275,975 Votes
13. Broward, FL: D+264,211 Votes
14. Cuyahoga, OH: D+256,581 Votes
15. San Francisco, CA: D+254,647 Votes
16. District of Columbia: D+245,689 Votes
17. Miami-Dade, FL: D+208,459 Votes
18. Middlesex, MA: D+204,483 Votes
19. Montgomery, MD: D+200,047 Votes
20. Multnomah, OR: D+199,585 Votes
21. Baltimore (City), MD: D+193,307 Votes
22. Hennepin, MN: D+183,909 Votes
23. Milwaukee, WI: D+177,514 Votes
24. DeKalb, GA: D+173,832 Votes
25. Essex, NJ: D+170,216 Votes
26. Suffolk, MA: D+163,897 Votes
27. Contra Costa, CA: D+154,307 Votes
28. Denver, CO: D+148,907 Votes
29. San Mateo, CA: D+133,329 Votes
30. Dane, WI: D+132,427 Votes
31. Franklin, OH: D+130,349 Votes
32. Fulton, GA: D+118,346 Votes
33. Honolulu, HI: D+115,888 Votes
34. Hudson, NJ: D+110,584 Votes
35. Dallas, TX: D+109,758 Votes
36. Fairfax, VA: D+108,500 Votes
37. Palm Beach, FL: D+102,253 Votes
38. Hartford, CT: D+101,401 Votes
39. Clark, NV: D+100,883 Votes
40. Mecklenburg, NC: D+100,594 Votes
41. Sonoma, CA: D+99,158 Votes
42. Orleans, LA: D+98,719 Votes
43. Ramsey, MN: D+98,138 Votes
44. Sacramento, CA: D+97,989 Votes
45. Westchester, NY: D+97,663 Votes
46. Shelby, TN: D+96,794 Votes
47. St. Louis (City), MO: D+95,837 Votes
48. Travis, TX: D+92,636 Votes
49. Allegheny, PA: D+90,648 Votes
50. San Diego, CA: D+90,231 Votes
Republicans: 1. Maricopa, AZ: R+147,597 Votes
2. Utah, UT: R+139,669 Votes
3. Montgomery, TX: R+105,049 Votes
4. Tarrant, TX: R+95,849 Votes
5. Collin, TX: R+95,473 Votes
6. Waukesha, WI: R+84,019 Votes
7. Salt Lake, UT: R+77,664 Votes
8. Denton, TX: R+76,601 Votes
9. Davis, UT: R+74,972 Votes
10. Orange, CA: R+69,892 Votes
11. Tulsa, OK: R+62,318 Votes
12. El Paso, CO: R+59,133 Votes
13. St. Tammany, LA: R+58,995 Votes
14. Cherokee, GA: R+56,673 Votes
15. Greenville, SC: R+53,615 Votes
16. Shelby, AL: R+51,385 Votes
17. Forsyth, GA: R+51,337 Votes
18. Knox, TN: R+50,308 Votes
19. Johnson, KS: R+47,875 Votes
20. Baldwin, AL: R+47,592 Votes
21. Hamilton, IN: R+46,951 Votes
22. Okaloosa, FL: R+46,747 Votes
23. Ottawa, MI: R+45,429 Votes
24. Collier, FL: R+44,822 Votes
25. Williamson, TN: R+44,708 Votes
26. Clay, FL: R+44,263 Votes
27. Ocean, NJ: R+44,209 Votes
28. Lee, FL: R+44,006 Votes
29. Warren, OH: R+43,652 Votes
30. St. Johns, FL: R+43,323 Votes
31. Douglas, CO: R+43,303 Votes
32. Butler, OH: R+42,788 Votes
33. Oklahoma, OK: R+42,746 Votes
34. Lexington, SC: R+42,514 Votes
35. Lancaster, PA: R+42,188 Votes
36. Santa Rosa, FL: R+40,418 Votes
37. Westmoreland, PA: R+40,210 Votes
38. York, PA: R+40,113 Votes
39. St. Charles, MO: R+38,946 Votes
40. Cobb, GA: R+38,598 Votes
41. Livingston, LA: R+38,062 Votes
42. Lubbock, TX: R+37,198 Votes
43. Kern, CA: R+37,123 Votes
44. Brazoria, TX: R+36,441 Votes
45. Washington, UT: R+36,361 Votes
46. Brevard, FL: R+36,307 Votes
47. Smith, TX: R+35,875 Votes
48. Williamson, TX: R+35,131 Votes
49. Bay, FL: R+34,825 Votes
50. Sedgewick, KS: R+34,529 Votes