wow, this is a tough decision... i pretty much like them all cause the style of soloing/riffing is so signature... but heres my 3 favorites where im just like "That pretty much killed my face."
1) Convergence - since forgotten: the first 10 seconds of this solo is probably my favorite solo intro ever.
2. Obituary - chopped in half: classic
3. Disincarnate - Confine of Shadows: the way this solo kicks in is SO sick with the harmonizing, ive probably listented to this and the first one about 1,000 times, rewinding back to the start of the solo
ive never really listened to a lot of the testament stuff so ill have to check out the solos everyone mentioned.