Actually, I think that in ANH, if Obi Wan went off to fight Vader (imagine they didn't need him to take down the tractor beam, and he wasn't distracting Vader, so he went off just to fight), it would still have been a great fight. So I don't think it's a great fight because Obi Wan was helping them escape.

It must be down to intangible factors - maybe the whole production style, the music, the acting, the pacing - that makes the ANH fight great, and the Menace one mediocre, despite the film-makers' best efforts to make it engaging.