You're missing out on the best parts of Vertigo if you haven't made it past 12, the best maps in the campaign start popping up around 13 - 18.
I'm not giving Interplay any more money.
My favorite D1/D2/D3 level (or levels)? My favorite is probably the original mission set in each. Aside the the multi-player and fan made levels, it's the original mission set I come back for time and time again... and also anything by Ron Lester (in Descent 3).