I think Quicksilver potion is too good to pass up on a Carnosaur if you're putting an item on it. Guaranteeing that you hit first one time can be critical, and attacking with two units before your opponent gets to pick a unit on your turn can be game changing. Against another monster, you'll be able to put a load of wounds on it before it even swings.
The Oldblood command ability bonus attacks give more to the Carnosaur's bites than to the Greatblade if you don't have a relic blade, because they hit on 3+, as opposed to 4+, but if you do have a relic blade, you'd get more out of the base extra attacks from the warblade/war spear than the Greatblade (so you still put the roar bonus attacks on the carnosaur bites), so they are better in pretty much all situations, being more reliable with the extra attacks, with the same potential damage or more.
Even with the extra attacks from the Oldblood, the warblade/spear are giving more potential damage anyway (16 vs 15); you'd need legendary fighter on the regular carnosaur for it to match, but that would mean not having the OB command ability, unless you are running a bloodclaw starhost, in which case, why not run a second Oldblood on carnosaur instead for +4 attacks? So basically, the only time the greatblade is better is with a relic blade when you are running a bloodclaw starhost with 2 Oldbloods on carnosaurs, in which case, you'd probably be wanting the oldbloods to have quicksilver potions anyway.
Unless the target has a save of 4+ or better, the normal blade is the best, with the weapons all being equal against 4+ saves.
Considering most things have a save of 4+ or worse, I'd go with the warblade most of the time, unless you are specializing it to kill those units that do have 3+ or 2+ saves, in which case, the spear and Greatblade are better.
