Helen’s favorite structure is:

G-Protein Coupled Receptor Kinase 2

Why?

G-protein coupled receptor kinase 2 is an up and coming target for heart failure. It's crystal structure is useful for those of use working on designing inhibitors as therapeutics for heart failure. Mutagenesis studies have
shown that targeting specific amino acids is difficult. The crystal structure is very important as it is the entire conformation of the protein that we need to understand and the crystal structure helps to reveal this.—Helen

Caren Sullivan’s favorite structure is:

Bismuth

Bismuth

Source: Theodore Gray

Why?

A thin oxide layer results in a beautiful iridescent surface coating the bismuth crystals. Bismuth has a relatively low melting point, so you can melt it down and make these crystals at home in steel cookware! They're relatively easy to make, but also contain incredibly complex and mysterious crystal structures! One of Nature's most beautiful creations!—Caren Sullivan

Allison Fleshman’s favorite structure is:

Poly(ethylene oxide)3:LiCF3SO3

PEOLiTf_Zardalidis

Source: Zardalidis, et al. Macromolecules, 2013, 46,
2705-2714

Why?

The phase diagram of the 3:1 compound of  polyethylene oxide with lithium Trifluoromethanesulfonate is beautiful! It's also a common crystal in the polymer electrolyte world. I consider it an 'old friend'.
Allison Fleshman

Kranthi Kumar Namila’s favorite structure is:

N-acetyl L-cysteine

overlay

Source: CrystEngComm, 2013, 15, 6498

Why?

A novel polymorph of N-acetyl-L-cysteine (NAC) is discovered three decades after the first report on the X-ray crystal structure of this bioactive compound. The crystal structure of the new orthorhombic polymorph (form II in the P212121 space group) is characterized by X-ray diffraction and compared to the triclinic structure of form I (NALCYS02 and NALCYS10 in the P1 space group). Both polymorphs contain a C(7) chain of COOH…OLC–CH3 hydrogen bonds, except that the COOH group is rotated by 180u in form II
to make an auxiliary C–H…O interaction with the methyl group in a R2 2(8) ring motif. The known form I contains weak S–H…OLC–OH and N–H…S hydrogen bonds whereas the new form II has only N–H…S interactions.—Kranthi Kumar Namila

Sudalai Kumar S’s favorite structure is:

Water-Ice

Water-Ice

Source: University of Hyderabad

Why?

Perfect tetrahedral arrangements of water molecules through hydrogen bonding in the crystal lattice makes the the ice less denser than the liquid water. This is one of the peculiar behaviour of water in the solid state. The photo shown here is taken from University of Hyderabad during the hail storm in 4th March 2014. Photo Courtesy: HCU Students.—Sudalai Kumar S