The original party (PDS) fused with the WASG in the early 2000s which consisted of former SPD members who weren't happy with chancellor Schröder (SPD) and other members of government. Together they formed the Left party.

The Left is the successor of the former leading party of the German Democratic Republic. That's why they are much stronger in East Germany and almost non-existent in West Germany. They also aren't too critical when it comes to the previous undemocratic system in the GDR and what their former party has done. There exist a few even more extremist left parties in Germany but the Left isn't that far off either.

SPD slowly moved closer and closer towards the center and nowadays they aren't too different from CDU/CSU (specifically CDU). But I think it would be unfair to say that they share the same political view. The difference is quite big though as the Greens are usually considered to stand between the Left and SPD in the left spectrum.

SPD is more to the mid. Overall the differences are not big, but the Left often stands for pacifism in foreign relations and vote against military deployments.

If I were German, I'd probably vote for the Pirate Party (if it could become significant enough to get seats), The Left, and/or Greens (if they moved toward an Ecosocialist platform like the American Green Party.) Probably means I would vote for the Left even though something like the Pirate Party would more closely align with my views. But that is without considering strategic voting, which with an Mixed Member Electoral system seems a bit confusing.

Before the introduction of leveling seats a few years ago, it was strategically clever to vote for different parties with first and second vote, as this would maximize the effect of your vote. Now it is not that important anymore.

One strategic consideration though: It not only makes sense to vote for a party, if they clear the hurdle to gain seats (5%), but before you can help them to reach 0.5%, which is the hurdle for getting money for the votes and donations. Reaching this milestone really can help a small party getting financed and be able to grow.

I'll be rooting for AfD's ultimate demise and burial into the sad dark history pages of proto-fascism. 

Anyway, it looks to be SPD anyway. I just hope the Greens get into government.

All I know is that SPD consistently sat on the wrong side of important matters, like German reunification.

The SPD (together with the greens) was actually government, the first time germany was participating in a war after WWII. Also they changed the social systems to put more pressure on jobless people to create a sector of low-income work, a change which ultimately gave the SPD-members a lot of trouble and probably allowed the Left to grow into a substantial party. You could argue that these changes were successful, in germany being economically more successful, but also socially more divided. Maybe you could even argue the social division helped to establish the AfD.

But current SPD has changed quite a bit and tends to be more on the side of social security. They now stand for minimum wages for instance. So maybe a new SPD-green government might act differently this time around.

Sadly this probably won't happen. The AfD has established itself as anti-government I guess and as long as their is some sort of division between people (social class or anything else), the AfD probably could utilize it to find voters.

Well there are a lot of half-baked ideas like:

unconditional basic income (which would cost quite a bit more than the whole state generates in taxes).

abolishing commuting allowance (even though it's not exclusive to cars and would cause people to settle even more in already crowded areas which in turn will cause an increase in housing prices)

Expropriation of housing corporations for billions instead of using that money to create public housing projects.

Withdrawal from NATO.

Immediate coal phase-out (even though the power generated from renewable energy sources are insufficient at the moment and that won't change in the upcoming years. Furthermore, it was the Greens "fault" (SPD as well) to phase-out of nuclear power in the early 2000s. Doing a 360 would cost dozens of billions and take several years until the reactors could substitute coal power plants. And a few years later they will be phased-out again because renewable energy has finally taken over.)

Well that were a few of the more radical ideas. Obviously some points aren't shared by the majority of a party but at least a good portion of it. Others are all too real and are either about to happen (expropriation) or part of a party's manifesto (NATO withdrawal).

It's kinda strange, that in an answer to Rol you put NATO withdrawal as a radical idea. Because Rol is from Austria, and Austria isn't a NATO member. Are Austrians radicals?