Well I do not recall Katsuyu tanking a TBB. In addition, Katsuyu technically did not tank Chibaku tensei considering it was inside Naruto's Kyubi cloak at the time.
Also, could you please show me where Katsuyu's acid melts through Diamond and Platinum, or it is said that it can...as far as I recall it can easily melt through rock, but there is a vast difference in rock and diamond.
Upon re-reading the battle between the Sannin, Katsuyu's acid also appears to be rather slow moving, and even small to a degree....Manda was easily able to evade the attack and fast enough to entangle Katsuyu....would have bitten her had it now been for Gammabunta.
In addition, no agglutonation or undeground movement has occured from what we've seen.....while it is possible, based on the animal Katsuyu derives from, until we've actual seen it we cannot simply assume this is possible.
Overall, Katsuyu is the greatest defensive summon because it can take a good bit of damage as well as split up to escape attack. It is also fair to note that Katsuyu as a whole never took all of these attacks, but instead as a small portion of herself (especially chibaku tensei and Naruto's cloak). It is easily arguable that taking the direct brunt of shinra tensei as opposed to indirect blowback in a separated form can produce differing results.
Katsuyu has displayed no speed feats as a collective summon as opposed to a small individual unit (where speed has been seen).
I personally would venture that Ibuse is the best summon. Firstly it isn't so large as to be a walking target, but large enough to crush opponents. After that, Ibuse has demonstrated exceptional speed and navigation underground....capable of Comming up on Mifune (known for his speed) from below. Also, the poison toxin it has only had one known person with a cure (Chiyo), and only because she had survived several encounters with Hanzo.
Going on animalistic traits, Ibuse would be extremely fast in water (Hanzo was said to be unmatched in underwater speeds) as well as regenerate limbs.
With speed, underground stealth, size, and extremely lethal poison....I'd say Ibuse is a more dangerous combatant as well as more efficient for transportational uses; however, does not hold the same defensive capabilities as Katsuyu.
It all comes down to what one really wants in a summon to be honest.