With my 20+ years of 4x4 ownership here are my thoughts on the subject......
1. If you want absolute reliablility then street cred has to take a back step, I'm afraid, its Jap all the way.....The Nissan Patrol as AW8 mentioned was the utmost in reliability, I had one from new and it was amazing, was properly serviced but only ever needed a clutch plate, and that at 300K miles!! It towed like a dream and coped with the worst of the worst terrain. Toyota Landcruisers / Amazons have similar track records. Cons with these cars are that they are quite clumsy and lumberingly huge on UK roads, and mightly difficult to fit into town centres and car parks!!! Newer Nissans I have no experience of, but I know a lot of reliable XTrails, unexciting though they may be.....
Mitsubishi Shoguns / Pajeros don't have quite the same reliability index, though, there are quite a lot more electrickery-bits on them, just like X5s, Rangies etc.
2. I traded my Patrol in for a P38 Range Rover which was a one-owner special edition and I cossetted it for 6 years - I actually have not sold it yet, a tinge of sadness about getting rid of it plus time-constraints and was working away from home for a year. I loved the Rangie but it was by far the most expensive car maintenance wise. It cost an absolute fortune to keep it pristine, and at 11, nearly 12, years old and 136K miles later, it still runs superbly, although with the quirks that its marque is known for!!! It towed fantastically and never let me down, kept me mobile in all weathers, but OMG, the nuisance (expensive) electrical and aircon faults were legion. Also, the build quality is really rubbish next to German vehicles. I'm not enticed by an L322- older models are showing similar electrical catastrophies to the P38!!! Having said all that, my beloved Rangie has had all her problems ironed out and I'm looking for a good offer and a loving home for her!!!!!!
3. Tried the Disco - all three generations - absolutely hated it. 1 and 2 seemed so unbalanced, top-heavy - saw a few tumble over in accidents. 3 - well its just so bug-ugly, like the Cayenne really. And also plagued with electrical gremlins.
4. Cayenne was an option but I hated the looks and its not cheap to maintain either, at Porsches prices. It shares heritage with the Touareg, which I also considered (The V10) but was put off by tales of expensive failures and the hard seats.
5. Similarly, the Merc (boring) and Audi (bloated and too big for normal parking spots!) didn't do it for me. All the aforesaid vehicles can tow pretty capably with their large engines, it just depends on what interior spec you like, and what reliability index you believe!!!
X5 E53 haven't got the greatest reliability record, but in my opinion, none of the others has the same drive quality AND high quality finish. The Audi is beautifully finished, but the car is bloated and huge, unlike the sleek lines of the X5. And a V8, if cared for and (here's a controversial one!) not LPG converted, will last for many many thousands of miles- my Rangie is testament to that! The Patrol was a straight 6, so don't discount a 6-cylinder car, V6 or straight!!!
I owned a 3.0i for 9 months prior to part-exing for the 4.8, and the 3.0i was a really nice car, towed very capably, but the MPG when towing was not as good as the 4.8 gives me now!!! I get around 20-21mpg towing a big heavy trailer with a big roof box on the X, on the 4.8. I can't even feel the trailer to be honest.
Its got a factory fitted towbar which integrates with the electrics. Its such a nuisance to remove that I leave it attached and locked - protects my rear bumper from the idiots behind me!!! The other thing with X5s esp in the used market, is the BMW AUC warranty, which is as good as it gets really, with a decent name for no-quibble repairs under warranty unlike many others.
Anyhow, that's my ramblings after many years of 4x4 ownership........hope you make the right choice for yourself in the end, good luck in the search!!!!
PS forgot to mention, I don't enjoy diesels, so haven't mentioned them as I have no experience of ownership of one - having said that, the 3.0D X5 is of course by far the most popular model in the UK, so lots of people DO like them!!