Commanders are usually creatures of course, so you can even play it loosely in your deck if your commander has a strong trigger worth doubling, like Sheoldred, the Apocalypse or Voja, Jaws of the Conclave
The activated ability may be a somewhat difficult cost, but with cards like Timeless Lotus and Mobile Garrison, imagine how many additional combat phases you can get
Sylvan Advocate or Duskwatch Recruiter into Kozileks Return looked great, especially if the Duskwatch Recruiter transformed and was powering out a Reality Smasher on the next turn
Phyrexian Altar is also powerful because its a free sacrifice outlet that can be activated at instant speed as many times as you want
These cards are crucial to playing your most powerful cards
Generally seal is the best non-threat card you can play on turn one as it often lets you recover a tempo lead a turn or two down the line