"Get Happy" is the best little musical you'll find for miles around, with the songs of Harold Arlen polished to a glitter by five grand performers and limited to 100 customers a night.

The place is the new Starloft Cabaret, entered through the silo of Frank Matthew's Hayloft Dinner Theater in Manassas, Va. Matthews' aim is to make this nook of red-checked table-cloths a spot where two can have a date with food and drink for $20.

Arlen's remarkable career has brought him recognition as "the song-writers' songwriter." His lyricists are among the best: Ted Koehler, Jonny Mercer, Ira Gershwin, E. Y. "Yip" Harburg. Their collaborations include "Minnie the Moocher," "Stormy Weather," "Evalina," "Only a Paper Moon," "Blues in the Night," "Accentuate the Positive" and scores more.

Distinguiishing "Get Happy" is the seamless way its creaotors, star performer Richard Beneville and arranger-pianist Robert Tartaglia, have blended the songs. Without the "then-he-wrote" blah, they left the songs sing for themselves. Beneville, who starred recently in "Stop the World," has staged it in little space with vital flow.

Familiar through area dinner theaters, each singer is strikingly individual: dark, stylish Beverely Cosham, who's become a marvelously astute performer; blond Michele Mundell, still looking like "Miss Maryland" but with some new low notes to her soprano; tall, dark, handsome baritone Larry Beckler, who's learned to unbend; savvy Benneville, who makes a Playlet out of "One for My Baby." But all are individuals secondly, the effect is pure ensemble.

Performances start nightly at 8, Mondays excepted. The left opens at 6 with sandwiches and "small dinner" priced from 50 cents to $4.50. Entry fee is $5. Reservations, held to 7.30, at 631-0230.