Well, if you can afford equipment that gives you Greater Magic Fang, I suggest the direct Shifter Route - otherwise, consider taking some more Druid levels for more castings of Greater Magic Fang. (4+ 3rd level slots and some 4th level slots, and a greater "+" when casting it)
(My own Shifter is Druid8/Shifter9 at this time, started at Druid8/Shifter5, if I remember correctly)
For some of the special abilities:
Regeneration does only help you against damage you take in the form you currently are, because it makes regular damage to subdual damage and heals this subdual damage.
Fast Healing does heal every damage you have.
A limitation you could (and I do as a player) interprete from the rules is, that new forms cannot help you with damage or other penalties you suffered in earlier ones. (Remember, your hitpoints do not change when you choose a new form)
You gain Spell Resistance, because this is an extraordinary ability. For the same reason, you gain also Magic Immunity from Golems or the Will o`Whisp.
Since you gain every extraordinary ability, be careful - not all of these abilities are good for you (the Rakashas Vulnerability to blessed crossbow bolts, the Berserk ability of Golems)
It is unclear if the HD limit for forms still applies for Druid/Shifters(meaning you may never take forms that have more HD than you). I decided to take the HD limit - it forces me to be more creative with forms, instead of simply taking the most powerful one...
Really powerful forms:
The outsider ones, especially the celestials (look at Devils and Demons, laugh, and flip to the celestial pages
)
Giants (including Trolls) are great forms thanks to the incredible strength.
Golems are very powerful, and the Stone Golem has no severe weakness.
Speaking of Contruct Creatures, remember that you can take the form of many objects when you choose the Mimic (an Abberation) or the Animated Objects (all Constructs).
The Treant or the Tendricolous are awesome plant creatures - with some luck you can even swallow a Dragon with the last form
Legendary Apes and many of the other dire or greater animals have high strength and good abilities. (Even the "standard" Elephant can be very powerful
)
Flying Forms: Giant Eagle at day and Giant Owl at night.
(Consider taking the Improved Fly Feat)
Later forms may be some Dragons (if you need speed, not maneuverability), and maybe some outsiders (arrowhawks?). I can`t remember the best choice, I never really needed one of them.
Forms for fun: Aboleth (well, actually also useful in water), Bulette, Delver, Elephant. There are more of them, just look it up.
(If I have the choice, I prefer interesting or new ones before old but powerful forms
)
Be aware that many of these forms do use manufactered weapons, so have a good weapon handy. (My weapon of choice is a Surestriking Scimitar +1 of Frost, beeing upped to Holy version soon
)
Mustrum Ridcully
.
