deathmarks are nice, i love them,
but my favorite models are teh trairch and lychgaurd, although i like the jump infantry a bit more, but those dispersion shields are really funny, it makes some people think twice before shooting a volley of plasma fire into them.
the c'tan shards, could be usefull, but I'm not seeing it atm,
triarch stalker, wow, costs a bit, but a great model, heat ray combined with targeting relay i like it, flayed ones are nice, but i don't think i'll use them often