Just whatever you feel most closely relates to your ascent. Short crags with trad grades will see loads of Solo grades, whereas you sometimes see boulder problems, just as high, with E-grades that typically gets logged as "Sent". (Physical Graffiti at Dumbarton comes to mind here)
In reply to Will21: Ah Harborough rocks... If I used a pad I'll have logged it as sent, if I didn't I'll most likely log it as a solo if its in the guide as a route. Some of those easy aretes are very short though so maybe you're right. I guess boulderers didn't used to have pads.