Elizabeth Barrett Browning's Sonnets from the Portuguese is my favorite romantic poetry. The brevity of the sonnet form in general seems well suited to expressing romantic sentiment, allowing intense feeling to be maintained yet not be overwhelming. The rhyme pattern of the Petrarchan sonnet may especially lend itself to strong expression in the first eight lines with the less dense rhyme of the closing six lines perhaps encouraging a sense of lingering.
While my own tastes in verse runs counter to E.B.B.'s common use of enjambment, such perhaps gives her sonnets a softer and sadder feel.
However, these sonnets appeal so much to me largely because they come from a melancholy perspective and express the traditional aspects of romantic love such as unworthiness and self-sacrifice. For someone inclined toward depression, the idea that even even the lowest soul can be rescued by love can be very heartening.
The sentiments and imagery also seem to have substantial application to the Christian understanding of the relationship of Christ and his Church, portraying a king who subdues his beloved to himself for her good and glory.