Everything goes these days, but the classics remain classic. I assume by formal you mean black tie, tux, dinner jacket, etc. (interchangable terms). The topic has been done to death here. Boiled down:
SB or DB peak or shawl, black or midnight blue (really a bespoke option only). Grosgrain or satin lapel facings and stripe along trouser outer edge.
Wing (impractical and the RTW options all suck) or fold collar shirt, with either a pleated or a pique front (harder to find). Black self tied bow tie.
If SB coat, vest or cummerbund. If vest, low front 3-button formal vest, made from self (i.e., the body of the suit) cloth, or (to be approached with caution) white pique (with wing collar shirt only) or some fancy silk (dark red shantung can look nice). If DB coat, no vest or cummerbund.
Black socks, preferably silk. Black patent leather plain toe laceups or calf opera pumps with grosgrain bows.
If you do all that, you will probably find yourself in a minority comprising at most 30% of the guests at the average black tie event in Manhattan. Or, I should say, a plurality, because there will be so much variety among the other male guests that no single other grouping will look even close to consistent.