I'll start... I'm a fan of crossing a TB over pretty much any cold-blood or native, and my absolute favourite is the connie-TB cross, my best horse ever was out of a pure Connie mare and by the local old fashioned stamp of a TB, and he was a super example, everything you'd want in an overgrown pony, smart-looking, hardy, bold, agile and very very clever (and just sharp enough to keep a keen teenager on her toes!). I wish I could find another like him...
My least favourite is a pony on my yard who is a Shetland/Welsh D Cross and he's exactly the kind of nightmare you'd imagine if you take the worst qualities of those 2 breeds and combine them, he's 13hh of pure devilment (pretty though!). I'm also a bit wary of anything crossed with a shire, they seem oddly hot and sharp for a draught cross (I'm told that's possibly an issue with the pure shires too?) and prone to breaking down young too 
I think its interesting to compare the attitudes to crossing horse breeds with dogs, in the dog world cross breeding largely seems dictated by fashion, and generates so many ill bred, ill tempered sick dogs that anyone who just wants an ordinary family pet is strongly advised to stick to pedigrees only (on MN anyway!). Whereas judicious, deliberate crosses of horses seems to me to usually produce very nice, very useful animals for the amateur/leisure market, I just wish more people did it (the difficulties faced by small scale breeders could fill up a whole thread on its own I suspect)
So anyway, share your favourites please! 