Australia - given Hussey's ODI record at the time was 300-odd runs at 100 at a strike rate close to 100, I always thought the Aussie selectors shot themselves in the foot by not just adding him to the test party for the 2005 Ashes. Maybe would have been harsh on Katich, but the guy was in incredible form..
England - too many choices, but I think the worst in my time was picking Stewart to keep and leaving out Russell. Just so they could play some combination of Lewis-deFreitas-Emburey-Pringle or Cork-Croft at 7 and 8, while giving a dozen tests (combined) to the likes of Morris, James, Lathwell and Gallian and another 15 to Nick Knight.....
In the 1993 loss to Australia, England picked 24 players.. They picked TEN seam bowlers in a 6-match series - Bicknell, Caddick, de Freitas, Foster, Fraser, Illott, Lewis, Malcolm, McCague, Watkin. Maybe none quite rank as the worst selection, but clearly they were the worst selectors...
NZ - picking Lee Germon as keeper and forcing Parore to bat at 3... never giving Andre Adams a second test match somewhere.... and picking a test side in that awful 1990's period that I think had Dipak Patel at 5 and Chris Harris at 6....