Firebird all the way (I prefer CE, then Champion, then Star, then DX). I've used monsters but they aren't as predictable and I find it very hard to judge when and how far a monster will skip. But if you are using a monster, I like the feel and flight of the old pearly ones the best. Another advantage of the firebird is that they come in all weights, from 150-175. I keep a low 160s and a max-weight in my bag at all times.
As far as other innova overstable drivers go, I really do like the flight of the Pegasus. I stocked up on 7-8 of them after they quit making them, and usually have one or two in the bag. The old DX plastic feels great, and these discs are among the straightest low, wind-fighting fliers I know of. I would never use a Pegasus for distance, but I find them to be laser-accurate about 270' and in, both forehand or backhand. They don't need much height to work an 'S' and they fly straight in all but the strongest winds. They don't have a lot of side-to-side fade, so they land pretty close to the line you release them on, and the blunt edge takes a lot of beating before it becomes too flippy. Often the only true DX disc in my bag is a Pegasus.
Rams are hugely piggish and best used for rollers if you have the power to throw them. Vipers are much the same way -- hugely piggish, but with more glide and can sail forever on an anhyzer if you get them beat in enough (though they don't roll as well as Rams). I know of at least one hole that I've parked more than once with a Viper anhyzer that I've never been able to reach with any other disc.
Whippets and Banshees are great discs. For a long time they were 90% of what I threw (all forehand) off the teebox. These are also my two favorite hammer/thumber discs of all time. For upside down shots, I think they are more predictable and carry a better line than firebirds, monsters, orcs, etc... That said, they both fly fairly similarly for me. Whippets are more overstable when fresh but go farther when beat in. Banshees are a little shorter and a little less overstable overall, but are amazingly predictable. Keep in mind that the Millenium EXP1 is essentially the same as a banshee -- just a different plastic -- and a very nice feel to it.
If I had to recommend overstable Innova discs for players to try out, it would be the Firebird first, followed closely by the Banshee/EXP1...