Maverick ML8 frame for £700…
They're plenty burly enough for a bit of abuse, but my god they climb well too!
I've been a Maverick fan for a while (and I'm on my 2nd now), they're highly under-rated mainly cos a lot of people don't like their looks, and their proprietary shocks. The ML8 though is incredible, 6.5" of travel, just over 7lb frame weight, yet it would make a better all day XC bike than many 4" travel bikes, whilst still being able to take a 160mm fork and be chucked down some seriously rough stuff!
I've got a Durance (the lighter, shorter travel version) and a Giant Reign X that I use for messing about on, as well as a couple of hardtails, but if I could only have one bike… It would be an ML8 as it's the best compromise of everything IMO.
EDIT: Oh, and at 6ft tall, you'd probably want a large…