よりによって相手がノーマークのスーちゃんなんて。

。

Of all people, for it to be Su-chan, who nobody was even paying attention to.

Literal

Of-all-things partner [subject-が] no-mark [possessive-の] Su-chan [exclamation-なんて].

よりによって ('of all things, of all people') expresses dismayed disbelief at an unlikely coincidence. ノーマーク ('no mark') is from sports scouting — an unmonitored player — extended to mean someone nobody was watching or considering. なんて at the end adds incredulous emphasis. The sentence implies a romantic or competitive revelation.