BACKGROUND
I'd recommend these few as definate back crypts.
Cryptocoryne wendtii 'Mi Oya', pontederiifolia (spelling, rare-ish). Cryptocoryne balansae is my very favourite. Easy to grow and looks stunning at the back of the tank.
MIDGROUND
These i'd recommend as midground crypts that some could also be used toward the back.
Cryptocoryne becekttii, undulata, walkeri, wendtii (green, brown), x willisii.
FOREGROUND
Small Cryptocoryne that stay under 10cm.
Cryptocoryne parva. Stays very small and is very quaint.
Cryptocoryne wendtii 'tropica' is slightly bigger than parva, mainly due to its bushier growth.
Most crypts are very easy to grow. Certainly the ones i've listed are the easiest to keep. The great thing about crypts is they are so easy. They tolerate a wide range of temperatures, with most i've listed coping anywhere between 20-30C. They also grow in low-light set-ups, which is great because most people find they have inadequate light to grow alot of plants without going into CO2 + ferts.
Crypts do take time to adapt to a tank. So don't be alarmed if they appear to 'melt' away, this is normal. Its called crypt melt. The plant is basically altering itself to grow in your tanks conditions, given some time it'll grow back...hopefully bigger and better than before. I found with my various crypts i was gettinga new leaf every three days. So they can fill out quite quickly.
They don't take kindly to being moved once established. So plant them where you want them. I've killed off a few in my time because i've moved them too often and they havnt recovered.
Hope this helps,
James ( I LOVE CRYPTS ) hehe