Posted By:
Kevin SaucierYou are very welcome and I'm thankful I can help. I am also glad we are past the "honeymoon" stage" (aren't we?) 
. Here is the answer to your question. It's fairly easy:
Aside from the typical reprints (Ruth candy etc.) the three most commonly seen altered cards in my opinion are:
T206's - The thin stock is easy to work on and hard to detect without basic equipment...which many unfortunately don't have. They can come in a variety of sizes which offers no real consistency. Every part of the card is vulnerable.
Goudey's - The thick flat-edge stock, white borders and availability make these a prime target for trimming and whitening. For the most part they are much easier to detect but often slip by the graders. A favorite of many card doctor's.
Any pre-war HOF'er - Regardless of the issue these are a target simply because of the value.
Post war - 1955 Bowman's
On the funny side...the three most seen altered cards are:
All of Jim Cradell's collection 
Probably most of Mastro's cards
The T206 Wagner(s)
Kevin Saucier