If you're just talking about a simple straight Bb trombone, a whole lot of people (including Doug Elliott) think very highly of the Yamaha line, including models like the YSL 354 which is officially a "student model" but is used by a lot of pros and semi-pros. There are good ones on the used market, and often on Craig's List. Or see what Sam Ash stores have if there are any in your area.
If you're looking for a horn with an F-attachment, I'd recommend either buying a good used one (Yamaha, Conn, King, ...) or looking at the Wessex, Mack Brass, or the Packer that people have suggested.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)