Well I'm Croatian so I'm obviously cheering for my country. Football/soccer is by far the most popular sport over here (like in most other European countries). Croatia became an independent country in the 90s and debuted at the 1998 FIFA World Cup where we finished 3rd (memorable wins over Germany 3-0 in the QF and 2-1 over Holland in the bronze medal match) and the whole country went mad.
We have some very high quality players, with Luka Modric (won UEFA Champions League with Real Madrid this year) being the biggest and Ivan Rakitic being in the form of his life (won UEFA Europa League this year with Sevilla, just recently bought by FC Barcelona). Unfortunately we won't have our best attacker, Mario Mandzukic (Bayern Munich), for the first match, but I still think we can make a match out of it, Brazil could be very nervous since its their first match in front of the home crowd.
Other than my country, I'm always supportive of Portugal, Uruguay and Argentina and I'm always curious who does better in the Europe vs South America duels. African teams are also interesting as they usually have some really good players, but most of the times they lack discipline.
My predictions on who will advance:
Group A: Brazil and Croatia (Croatia-Mexico-Cameroon will be a close battle for 2nd, but I do believe CRO has slight advantage over the other two)
Group B: Spain and Holland (A bit predictable, Chile will be tough to beat though)
Group C: Greece and Ivory Coast (Can't see Japan passing and I do think Colombia will miss Falcao)
Group D: Uruguay and Italy (Italy and England are the big guns here, but URU finished 4th 4 years ago and they have the quality and continental advantage going for them and I see them going through at the cost of ITA or ENG, decided to eliminate ENG).
Group E: France and Ecuador (honestly one of the least interesting groups to me, Switzerland or Ecuador to battle it out for the 2nd place)
Group F: Argentina and Nigeria (ARG the obvious pick, would love to see Bosnia & Herzegovina pass the group in their WC debut, but I'm giving slight advantage to Nigeria)
Group G: Germany and Portugal (The obvious picks, watch out for Ghana though)
Group H: Belgium and Algeria (I expect Belgium to finish first and I'm going with Algeria as a surprise second over Russia and Korea)
EDIT: URU finished 4th 4 years ago, not 3rd (GER was 3rd). Sorry.