Infantry clearing jacks

Destroyers can be good situationally as a backup options.

Well, it does have that 4" AOE. A single 4" AOE won't kill that many infantry models, but the more infantry on the board the more tightly clustered they'll be, plus (depending on the scenario, your list, your strategy, the enemy list, the enemy strategy; you know, the game) you'll probably get to fire it two or three times before you get engaged or choose to charge your Destroyer into melee. Plus some casters can get you extra ranged attacks with the Destroyer.

BTW a 4" AOE is the same size as the Kodiak's cloud, for what that's worth, plus you can fire from farther away, possibly from being range of effective repercussion, and the Kodiak won't be inside the cloud taking up room. But of course it's harder to position it exactly where you want, and you can't do anything else that turn unlike the Kodiak... it's different, but not necessarily worse at killing infantry - which is the question being asked, so other benefits of taking a Kodiak don't rule out the Destroyer from this discussion.

Of course if the Destroyer is worth mentioning then so is the Behemoth; twice as many Bombards plus Powerful Shot means more killing per point than the Destroyer.

The Demolisher also gets two AOEs; they are smaller and have shorter range, but you can fire them when engaged so that can be good.
EDIT: did not notice the "melee" stipulation first time I read the original post. Sorry about that.

Black Ivan does have Point Blank on his Bombard, so... I dunno, he still looks too expensive and too pillow fisted to ever take seriously for me.

Mad Dogs get those boosted trample attacks. If only they were cheaper or less crappy overall...

Berserkers have Berserk, I guess PP wants us to use them against infantry? Even Drago is nothing special there though; take Yuri or Fenris or Doomreavers instead.

Also, if you don't mind Mercs, then you can bring a marshaled Mangler for a 2", MAT 8, Chain-Weapon Thresher (as well as a marshal-specific benefit such as Reposition) for a few points less than Beast 09.

Theoretically Drago is also decent in that role, but of course you can only use it with Vlad.

The Berserker can also work, but you need something to allow it to actually hit stuff in the first place.

I want to try Drago under Strakhov. MAT 9 Berserk is better (understatement...) than MAT 5 Berserk, and the option of Overrunning into a new knot of infantry (or just running away) after you've finished killing the first extends the use significantly . Plus the possibility of being DEF 15 with Poltergeist sounds entertaining (if you use Overrun on another model to move Drago into range you might be able to avoid spending the focus on charging and get to use it on the imprint instead).

I don't think it's enough to be worth bringing in for competitive games, but it's worth trying in a casual game.

It's kind of an odd question. Jacks are expensive single models whose main benefit in melee is their high base STR. But that benefit isn't required against most infantry. Jacks in melee are meant to be hitting other jacks or similarly tough targets. They only get lots of attacks through (expensive) special rules or the allocation of focus, which probably isn't cost efficient. And then there's the movement issue.

I'd put the Kodiak at the top of the list. Kodiak is best, as Vent Steam can occur at any point during activation and after clearing some infantry the Kodiak can still have a productive activation. Vent Steam + Trample is not bad as infantry clearing with a jack goes.

Beyond that, why do you ask?

Trample works quite well against melee infantry. They always get clumped together after charging something.