So when national parties run for European Parliament, they must choose a parliamentary group to sit with. The groups are:
European People's Party: Center-right Christian Democrats, Conservatives, and Liberal Conservatives. Strongly pro-EU and favor a "federal Europe." They're also supportive of low taxes, non-renewable energy, traditional family values, deregulation, stricter immigration policy, and opposed to Turkish EU membership. Largest members are Christian Democratic Union of Germany, Civic Platform of Poland, and Union for a Popular Movement of France.
Progressive Alliance of Socialists & Democrats: Center-left Social Democrats. In favor of further European integration, progressive taxation, individual rights (such as those of minorities), moderate immigration policy, and a large number are in favor of Tony Blair's "Third Way" ideology. Largest members are Democrats of Italy, Social Democrats of Germany, and Socialist Worker's Party of Spain.
European Conservatives & Reformists: Conservatives and Eurosceptics. In favor of economic liberalism (non-American definition), family values, low taxes, less bureaucracy, deregulation, and strict immigration policies. Largest members are Conservatives of U.K., Law & Justice of Poland, and Alternative for Germany.
Alliance of Liberals & Democrats for Europe: Liberals and centrists. The most pro-EU party; in favor of an eventual federal Europe. Also supports deregulation, low taxes, neoliberalism, preservation of civil liberties. They could be described as the European equivalents of moderate libertarians. Largest members are National Liberals of Romania, Democrats 66 of the Netherlands, and Democratic Movement of France.
European United Left-Nordic Green Left: Mostly democratic socialists, with some communists as well. Support heavy reforms to make the EU fully democratic and oppose neoliberalism. They support a minimum income, expansive welfare state, etc. Largest members are Coalition of the Radical Left of Greece, Sinn Fein of Ireland, and Communists of the Czech Republic.
Greens-European Free Alliance: An alliance of European green parties and regionalist and secessionist parties, like the Scottish Nationals. Support a green tax shift, renewable energy, social justice, and decentralization. Largest members are Alliance '90/The Greens of Germany, Europe Ecology-The Greens of France, and the Greens of Sweden.
Europe of Freedom and Democracy: Right-wing populists, mostly. Supportive of nationalism towards their respective countries, and opposed to the European Union and integration into it. Mostly right-wingers with the exception of the Five Star Movement. Largest members are U.K. Independence, Five Star Movement of Italy, and the Sweden Democrats.
European Alliance for Freedom: The "radical right." Opposed to the EU, and supportive social conservatism and tight immigration restrictions (total bans in some cases), and differ a lot by party on economics. Many advocate for banning religious displays by Muslims in public. They have been accused of xenophobia and fascism, but they denied the really extreme far-right parties (neo-nazis, essentially) from joining. Largest members are National Front of France, Freedom Party of Austria, and Party for Freedom of the Netherlands.
I personally prefer the Greens-EFA, but I sometimes support members of S&D or EUL-NGL.