Morph is a very situational skill. Depending on your team composition, the current situation you're in, and how you want your Adaptive Strike to act, your usage of morph will change.
In general, morph to strength if:
- You want survivability (more HP)
- If you are getting ganked (the extra HP will save you more often than you think)
- If you decide to build a tank
- You want a longer stun from Adaptive Strike (having 50% more Strength than Agility will give you the maximum stun duration)
In general, morph to agility if:
- You want DPS
- You want more damage from Adaptive Strike (having 50% more Agility than Strength will give you the maximum damage)
Swinging completely one way or another isn't that beneficial. Having lots of HP but no damage makes you an easily ignorable target, and having high damage but low HP makes you a glass cannon, and you will most likely be the first person to die in a team fight.
Foreword: I am focusing on DotA 2 since i stopped playing DotA 1 for quite a while now.
Offensive
Stuns will hold Morphling in place but wont interfere with his morph ability in any way.
Even Void's Chronosphere cannot stop his health gain.
Therefore you need a silence whether its an ability or an item to counter Morphling.
Important sidenote: Mantastyle does remove Orchid silence.
Naix with Orchid can be good because even if Morphling morphed into strength Naixwill deal more damage.
Clinkz with Orchid works well too because of his burst damage.
Silencer is very good because his Ultimate is not a target spell and global giving your team the opportunity to counter Morphling anytime.
(thanks to Decency for mentioning those 2 in his answer):
Ancient Apparation is another great pick vs Morphling because it prevents all kinds of healing including those from skills or items.
Lion's Mana Drain and Hex can destroy an illusion immediately. Combined with his long disable overall and his nuke he can be a threat at all times especially with Blink Dagger.
Defensive
To protect yourself against the enormous nuke of an Ethereal Blade-Morphling you should buy magic armor and a Mekansm. (Chen's Hand of God can be a substitute for Mekansm. But since you need to heal yourself quickly i would not rely on another players reaction. Having both is even better)
Furthermore BKB and spells like Repel help a lot.
Since EB-Morphling needs as much agility as possible to nuke hard he wont have much HP while nuking. This opens a window for a Blademail that can hit Morphling hard and enables the possibility for a good trade (your support for their midgame-carry)
Since Linkens Sphere is annoying you should pick heroes who can afford wasting a spell to disable his Linkens and have another good stun as a follow up.
For example: Bane, Lion, Shadow Shaman, Shadow Demon, Ogre, Rubik, Disruptor.
Best Answer
First of all shadow blade is easily countered(wards.dust...),especially if you play with experienced players.
People prefer blink dagger or force staff,both of which are similar to waweform so those items aren't too useful on morphling.
You might have idea to blink in or appear with shadow blade,do some damage and then escape using waweform,but remember that you should properly use your ultimate for that(escape-blink to the created illusion that you left in safe place).
Shotgun Morphling" focuses on getting an ethereal blade as soon as possible,buying shadow blade will just delay that and bring you only something you already have.
Shadow blade would work better with BKB/Satanic when you intend to hit you opponents rather than nuke them with magic.
Below you can see how you can surprise you enemies from a distance by waweforming and using ethernal blade(it has decent range),in addition you can use adaptive strike+morph agility(lots and lots of it),and then get away with you ultimate.