All can sound great on guitar. A simple air, played cleanly with accompaniment from another guitar or a harp can sound beautiful. I often play airs with single notes and leave the chords to an accompanying player. But you can also play airs with strummed full or partial chords. Melodies can be in first position or high up the neck.
Many approaches are possible.
Marches and strathspeys are fun on the guitar and some of these may be easier for players who haven't developed high-speed skills. On the other hand, some strathspeys are a challenge, with 16th notes played up the neck. Hornpipes are usually played slower than reels. That can make them easier. But some also have more complex melodies and many traditional players use more ornamentation on hornpipes.