Beauty is in the eye of the beholder and the same principle applies to determining the best love poems ever written. The style of some prose might be considered by some as literary genius but leave others totally cold. The best love poems are the ones that do something for you personally and this could relate to lengthy poetry or poems that are short and to the point.
Adding music to poetry often adds something you don't get with just words. An example of this is the poetry of Robert Burns the Scots bard who wrote many a love poem. When the words " My love is like a red, red rose" are set to music, it has more power than the simple poem on its own.
Those who love poetry and take the time to write their own will probably look at how a poem is constructed and make judgments on this basis as to the merits of the ode. Others who are not particularly poetry minded will possibly look only at the words and sentiments in judging those they consider to be the best.
For those who are on the receiving end of a love poem, the words will obviously mean that much more and it often won't matter too much what the quality is like. Even a simple poem scribbled on a Valentine card or an anniversary card will probably be more meaningful than one read in a book and composed by a well-known and talented poet.
Poetry is not everyone's cup of tea and often contain the nonsensical ramblings of people who think they suddenly become a poet by throwing together a bunch of meaningless words that just happen to rhyme with one another. Poetry can be good or bad depending on your personal taste and what one person considers a real masterpiece of prose could be considered as abject nonsense by another.
Those who spend their time writing and reading poetry are possibly in a better position to judge the merits and demerits of a particular love poem. Though it is not a strong point for some, reading poetry is something we have all done at some time or another and probably we all have the ability to determine what is good and what is bad.
Love poetry is a brand that is enjoyed by the romantics at heart, people who are either dreamily in love or who would dearly love to be. Choosing the best poem is like choosing the best song, it is simply a matter of personal taste and the one thing we can be sure of is that not everyone will agree on what love poem is the best.