The qualification stage isn't over yet but as of now, here are countries that are in the top 2 of their group (or top of their group, it depends on the confederation) and can potentially be participating in the 2018 FIFA World Cup.
AFC (Asian Football Confederation) [4 or 5 places in the finals]
Iran
Uzbekistan
Saudi Arabia
Australia
(South Korea & Japan move into the next round)
CAF (Confederation of African Football) [5 places in the finals]
Democratic Republic of Congo
Nigeria
Ivory Coast
Senegal
Egypt
CONCACAF (Confederation of North, Central American and Caribbean Association Football) [3 or 4 places in the finals]
Mexico
Honduras
Costa Rica
Panama
United States
Trinidad & Tobago
(all these countries have advanced to the next round)
CONMEBOL (South American Football Confederation) [4 or 5 places in the finals]
Brazil
Uruguay
Ecuador
Colombia
(Argentina advances to inter-confederation playoffs)
OFC (Oceania Football Confederation) [0 or 1 places in the finals]
First matches are to be played November 2016 so there is no current ranking for this confederation
UEFA (Union of European Football Associations) [14 places in the finals, 13 places left because Russia is automatically qualified as it is the host country]
France
Switzerland
Germany
Serbia
Montenegro
England
Spain
Belgium
Croatia
(Italy, Iceland, Greece, Sweden, Poland, Ireland, Azerbaijan & Portugal all advance to second round of playoffs)