I dig my Senn HD600 and HD650 'phones most. Yes, a good headphone amp extracts the nth degree of optimality with them, but I'm totally happy just plugging them directly into whatever the source happens to be. Often that's one of the recorders I'm currently using- DR2d, DR680, R-44, and frequently I'm driving both at the same time through a simple 'Y' cable so a friend can also listen.
I picked up the 650s new for $250 on sale about 9 years ago, and the 600's secondhand for less than that (can't recall) from another member here more recently. The HD600s are completely stock. I modified the 650's a bit early on, replacing the foam over the drivers with thinner polyester stocking material, and extracting a small piece of damping foam from the center of the driver cup (a non-reversible mod) which in combination make a subtle but to me significant tweak in slightly greater clarity and presence which puts them right where I like them to be. And those attributes also describe the difference I hear between the 600s and the 650s as well. The 600s are very nicely balanced and something of a standard reference headphone for good reason. I do enjoy the slight bottom and top end extension of the 650's though, which often makes for the best personal listening fit for my own recordings. I should replace the headband cushion foam on the 650's which is now totally flat, but I'm totally happy listening to either.
The other primary phones I have are universal fit in-ears suitable for environments requiring isolation, or when a smaller more portable solution is required-
I've two sets of Etymotic ER4 I've had for 15 or more years which I like alot but are currently packed away. I use them with my own modified expanding foam earplugs which maximize their environmental isolation and low frequency extension. In overall sound quality in a quiet environment they are a step down from the open back Senns, but not by much. They are my highly-isolating in-ear reference, but less convenient due to needing the foam plugs. I think those were about $250 a pair at the time, which seems to be my sweet spot for "good" headphones.
The in-ear set I currently use frequently is a set of Audio Technica ATH-ANC3 active noise cancellation in-ears with silicone flange tips. Those I use frequently for travel, in the office for listening on the computer or whatever. Not quite the same sound quality as the Senns (or the Etys as I recall), but very good overall and I've been quite pleased with them. Their active cancellation is excellent in reducing environmental rumble significantly beyond the isolation capability of the silicone tips alone (HVAC, car/plane noise, etc). Somewhat unusual to find active cancellation in an in-ear, but I chose them because of that feature and am quite happy with it. I originally had high hopes in using them as "active ear-plugs" at loud concerts with their active noise cancellation reducing low bass and subwoofer leakage beyond the capability of even well-fitted foam earplugs alone, but unfortunately the cancellation circuit doesn't have sufficient headroom capability for those kinds of levels and clips when overstressed in a situation where that functionality would be useful. I've thought of finding another pair and trying to modify the circuit, as I think it has potential as an audiophile ear-plug solution for concert hearing protection. But that's a back burner project that's long been on hold. I think I originally picked them up from Sound Professionals for $60 or $70 along with an original R-09 back in '06 or '07 or so.
I've never tried any of the the multi-driver IEMs, either universal or custom fitted.
I've also not tried any Adez, LCD, or HiFiMan open-back circumaural phones, although I'd like to, all of which came to market surfaced after search for serious phones which lead me to the Senns. At the time, I didn't care for the AKGs I tried which sounded over bright, or most Grados which sounded over-present and were uncomfortable. I really loved the Stax phones I tried, but didn't want to deal with electrostatics and the price was out of my range.