Depends on what Steak cut it is,
Ribeye i adore anything from Medium rare up to medium well (the fat keeps it tender) but i do tend to edge towards MR depending on the thickness and how big the vein of fat in the middle is
Sirlion i prefer medium, i like that meaty bite in my mouth, but if i get it right and get it perfectly pink inside, its still buttery tender Yum.
Rump... well i bought a piece of rump the other week for my birthday and i usually go medium well for rump, but this piece was literally marbled like a piece of Wagyu beef, so decided to cook it a little less to medium rare, and i can promise you it was the best piece of steak i've ever had(been to the Hawksmoor and Goodmans in London), if anyone lives up West Yorkshire way, go and try some rump (or sirloin) from Cryer and Stotts Samual valantine farmshop in Allerton Bywater, nicest i've had and it's not overly expensive either.
As for Fillet, no matter what, i am ALWAYS disappointed in Fillet steak, yes its Tender, but where is the taste? compared to almost every other steak it has the least flavour and i'm including the cheap cuts in there aswell, but if i was to have it, i don't think you can have it anything but rare because anything above Rare turns it into a flavourless tough piece of rubbish steak (may be exaggerating a tad)
But one steak that i haven't seen mentioned on here yet which is a real treat for price and quality is Flat iron, it isn't sold allot in any of the major outlets because when butchered it has a vein of tough sinew (i think thats the word) which needs to be cut out of the steak seperating the two pieces of steak, but if it comes preprepared or already butchered it is a great well marbled steak, tender and easy to cook well, cheap as chips, i always place it in my head as inbetween Sirloin and Rump, but with an almost Fillet tenderness, if you can see it, get some.