Manhattan is the hottest market in the world for retail real estate and that is good news for North Jersey as well, according to experts gathering in New York this week for three days of meetings, making contacts, and signing store leases. Rents on Fifth Avenue in midtown Manhattan are the highest of any major city, at $3,500 a square foot, and the attractiveness of the New York market has a spillover effect for North Jersey and the rest of the metropolitan region, said Gene Spiegelman, vice chairman of retail services for the international retail group at the brokerage firm Cushman & Wakefield.