What kind of 3D printing filament should I buy?

I really don't believe 3D printing filaments are perfect, but I will tell you what I like about them. The best thing about 3D printing filaments are the big choice of colors and materials.
The "normal" 3D printing filaments like ABS and PLA has a lot of colors, but what I really like the most are the special filaments like

what is abs filament : This is what lego bricks are made from. Slightly flexible, pretty strong. A little tricky to print with: releases not-so-pleasant gases when printed, is very subject to warping (so usually needs a printed bed).

PLA filament: This is the up-and-coming champion, especially for beginners. Pleasant smell when printed, cheap (about $22 a kg), easy to print, made from corn. Only issues really are brittleness and doesn't do well with continual exposure to the elements (since it's biodegradable).

PETG copolymer: This is a really nice material to print with -- it's got a nice gloss to it, releases no toxic gases and is stronger and more flexible than PLA, but without the issues of ABS. Some refined versions of PETG do better e.g. Colorfabb XT, Taulman n-vent. Bit more expensive than PLA.

Carbon Fiber PLA

Conductive PLA

Magnetic Iron PLA

Nylon: The strength king! Printed nylon is a little bit flexible, but super super strong. It's a pain to print with -- needs a hot bed, doesn't stick to the bed.

HIPS filament (High Impact Polystyrene)

PVA printinh material (Polyvinyl Alcohol)

Anything that can be turned into a paste or semi fluid material can be used to "print". Concrete, chocolate, epoxy, sla-resins, ceramics, pulp, wax, and dough are currently being used.
There are metals that can be powdered and printed with a laser in layers like titanium, steel, aluminum, gold, copper, and bronze. 3d printing andhra pradesh orissa jharkhand mumbai pune india