G for me as well–always. I do think that a nice open E sounds best on the electric though.
Usually a D chord. If the guitar’s out of tune, D will seem to make that more evident than most any other chord will (I don’t know why).
Because standard tuning on a guitar needs to be tempered, so the strings are not each tune *exactly *to pitch - instead they are each off just enough so that most chords sounds mostly in tune at most places on the neck. It just so happens that open D and G chords are the ones in that first group of chords folks learn that highlight whether your tune-up job is a bit too exact - i.e., your A’s and E’s sound great, but your D’s and G’s sound a bit off…you need to get every chord just a bit off, so they all sound usable…
That is a key element of what differentiates and truly well-made guitar - the whole system works together so than when you get the guitar in “tune” it’s natural resonances reinforce the correct harmonics so that the chords sound more in tune. This is also why there are alternate tuning approaches out there, such as a the Buzz Feiten tuning system, which changes the location of the guitar nut and requires that you tune the guitar a certain way so the temperament across the strings is proportional.
Sorry - geek hat off…
Come on, admit it. That hat never comes off. 
Acoustic, E major. Electric, E minor power chord.
E power chord, a string 7th fret. Then quickly jumping around power different frets descending down to low E. Then a shitty attempt at an A-minor 5 string sweep arpeggio at the 12th fret. 
So true. Drives my wife freakin’ crazy.
…the other day, while working out, I heard two folks discussing Ike Turner’s recent demise and they were getting the info on Tina all wrong; I said “Hello, sorry to interrupt, but when Ike found her, he’d already written what is now considered the first rock song and she was a 17-year-old kid from Nutbush named Anna Mae Bullock…” and realized I had lost them at Hello…
::sigh:: all the more reason to love the Dope!!
You know you’re a true music geek when your score on the bar top game music trivia is so high that nobody will ever beat it, and it only cost you $1 to get there.
Am I the only one that is boring and plays C?
I usually play a progression of C-F-C-G-C when I first pick up the guitar.
I hit my chord just to see if the guitar needs any tune tinkering. Then I’ll rip through some major pentatonic scales as a warm up and hit all the standard E-A-D-C-F-B chords a few times to limber up the hand.
After that I’m ready for anything.
Which guitar
Standard tuned electric, heavy strings - cheater’s A followed by open D and E, back to A. This is definitely the default action on picking up a guitar, however if I’ve picked up a guitar for a particular reason. . .
Standard tuned electric, light strings - probably not a chord at all, maybe a sweep Am at 12th fret, a (possibly) cleaner version of what Flander is up to.
Accoustic: open G - I’m probably going to fall into playing One Man Guy anyway.
Down a semitone standard tuning. Open E5 - think the opening chord from Live and Dangerous.
Drop D town a tone, intro to either Prison Song or Rock Lobster.
Bass, I think it’s C. Whatever key One of these Days is in.
I think that covers it.
I usually check to make sure it’s in tune (relative tuning on the 5th fret), then I play a G with the extra note on the B string, then a D to make sure the top strings are in tune.
I don’t habitually start with a chord.
Lately…
With an electric guitar, I typically tune the strings (using a foot pedal tuner) and then play the ‘Stairway to Heaven’ solo or the ‘Hotel California’ outro as a warm up.
With an acoustic, I typically the tune strings and warm up with The Who’s ‘Behind Blue Eyes’ followed by Led Zeppelin’s ‘Over The Hills and Far Away’ or, if I’m finger picking, The Beatles ‘Blackbird’ followed by Heart’s ‘Crazy on You’.
I usually play an open G. I think a lot of guitarists instinctively play that one first. I don’t know if it’s because it’s often the first chord they ever learned or because it has a lot of open strings an is a good way to quick check the tuning. It’s probably a combination of both. I tend to go through a sequence of open chords to check the tuning but I frequently start with a G.
I’ve gotten a little better at them, but I still tend to produce a lot of string noise when I play fast. Long solution, practice slower. Quick answer, tie a sock around the neck to dampen the strings! 
I swear it makes me sound like a pro!
A minor, and not strummed – fingerpicked, with a hammer (all emo and folksingery).
Wow, doesn’t that, like, break your guitar?
Nope. I play a C then A-E, then D-G (with a real brief sus2 in between, of course).
It’s also necessary to do some kind of sus4 trill anytime you hit an open D, of course.
“some kind”?? Nay, sir - there is only ONE kind: Boston’s More Than a Feeling! Right up there with Stairway in the category of “Songs my sister wanted to kill me over due to my practicing when I was 15…”
(yeah, I know, now you have to get that song bleached out of your brain…)