My pet card is the commander of my pet deck:
Merieke Ri Berit. I am aware that there are probably better options out there for a high power deck but they're not on flavour for untapping since MRB is the only* Esper commander that cares about being untapped. Love the art, love the mechanic (it's a bit broken, not OP broken), love the card overall. When I play her I feel like I'm asking for this lady of the woods to come and
lure stranded travellers to an embrace of death. Great stuff!
*There's also
Lady Evangela but she's even less powerful.
EDIT: I also like
Reality Ripple and I play it in some of my decks. Phasing is a surprisingly powerful mechanic allowing me to remove threats for an entire rotation or it protects my stuff from wipes and targeted removal for a rotation. The good thing about it is that it also protects stuff that care about imprint because the target doesn't leave the battlefield.