I've always picked my Sennheiser e609 Silver since I got it, combined with a 57, and two 990s. Some may think it is over kill but I love the results. I have the 990 on the back, 990 on one speaker, 57 off axis, and e609 right against the grill. But as for a single mic, go with the Senn. Placement, just right up on the grill, find the right spot for it.
First time I've read on here about an HR user using the 990s on cab. I first read about using the 990 in Electronic Musician a while back, but never heard of that since...
All the prior suggestions are good. But the typical person asking this question here is looking for one mic that is economicaly priced and given those limits I would go with the E609 Silver. With these going for only $99 from various sources they are a fantastic value.