Best headphones evar? Sennheiser HD-25’s
Mmh. This all seems to be a very subjective thing. I didn’t like those at all. Sounded somehow very hifi to me. To much bass. Not that flat. But they surely sounded ‘good’.
AKG K271 MKII
I’ve those, too. But i prefer my hd 650 more, because i like the sound and you can do longer sessions with it.
Agree with a lot of people who have shared the fact about using Sennheiser headphones. They are quite good at about 100 USD but then if you are not in a position to spend that much, the I would suggest a much cheaper option in Klipsch R6 headphones( https://baybloorradio.com/headphones/in-ear?product_id=1824 )which are not only cheap but are of exceptional quality. I have been using it for like more than a year and it hasn’t given me any headaches as such. Hence I guess that would be the best possible option for you as far as I am concerned.
Ignoring the fact that every sound engineer would never recommand to mix/master on headphones but to verify some details, Senheiser HD600 are definitly the best to mix or master if you have a silent work room (they are open).
They have a an optimal frequency response in basses which is the hardest area to solve. Everything from 35 to 200hz is clearly there without notch.
Basses freqs. are not overboosted has many other sets.
They have a wide open soundstage like AKG k240 or K712. The K712 are more comfortable but they are way too much boosted in the basses.
The HD600 are listed 400$ US in new york which is a good price if you compare with a speaker set that would be 4500+…
These heaphones are ugly as f*** but really the heaven to work. My exhaustives researches and compare tests conclued on HD600, I’m completly sold to them as you can see.
Sony MDR-7506 or MDR-V6 if you’re really on a budget. Very comfortable for long periods, reasonable isolation, don’t flatter the source material. Rugged, cheap and folding for easy transport.
For mixing on headphones, open-ended are a must so mixing with earbuds must be exceptionally challenging, especially if they are not meant for making music but just listening to it. Depending on how much on a budget you are, one option that’s recommended everywhere are Samson SR850 headphones. They are open-ended and beat most $100-$150 even though they are half the price! (in Poland they cost the equivalent of $35 dollars actually, on US Amazon they’re $50) Together with one Sennheiser model (don’t remember the name), they are the most recommended and highest ranked and reviewed low-end budget headphones when it comes to transparency to price ratio. So if you only want to spend $150 on new headphones, maybe just spend 1/3 of that sum and you can buy two more once these die on you 
I have those and I am quite happy with them. They are quite flat compared to others.
About the akg k271 mk2
Me too ,had them for 2 years verry dissapointed about te build quality …
Former batch was made in austria , nowadays nothing but plastic crap.
I went sennheiser hd …highly recommend
I love my Beyer 880’s but left earcan is starting to make funny noises. Would love to get my hands on one of these some time:
https://www.audeze.com/products/el-8-collection/el-8-open-back
About the akg k271 mk2
Me too ,had them for 2 years verry dissapointed about te build quality …
Former batch was made in austria , nowadays nothing but plastic crap.
I went sennheiser hd …highly recommend
Older products from AKG seem so extremely solid and well built, it’s a shame they turned crap.
I have an old AKG microphone (same model as ABBA used in the 70s :P), the thing can’t get broken, i bet you can drive a car over it and it will still work. It’s like an SM58 with kevlar armor. 
This seems to be what happens to most European companies when production is moved to Asia, such a shame. They produce stuff that breaks easily so you have to buy new ones all the time.