While Giuliani did write some excellent works for solo guitar, his best works imo are for guitar with other instruments. The guitar concertos have already been mentioned above. One of my favorite Giuliani works is his 'Gran Duetto Concertante, Op. 52", here with Lorenzo Micheli on guitar:
For solo guitar I recommend his three Sonatine Op. 71, not too difficult but musically more rewarding than some of his more technical pieces imo.