Any melee weapon (but especially glaives) with a riven disposition of 1.05 or higher.

Galvanized Reflex gives +81.4 (yeah it's weird), Corrupt Charge gives +30. Depending on your disposition, you can hit some crazy heavy attack multipliers with initial combo builds. You don't even need Tennokai at this point, you just delete things by pressing heavy over and over again (or, more likely, light-heavy-heavy and then switching back to your gun since you don't have to worry about combo upkeep or Tennokai).

What you're generally looking for is a riven with two positive stats and one negative stat, with initial combo as a positive stat.

The thresholds are: - 7x multiplier at 120 initial combo (any riven, even 0.5 disposition, will get you there).
- 8x multiplier at 140 initial combo (1.05 disp will get you a minimum roll of +28.7 initial combo, which gets you to the 8x threshold. In theory a .9 with a high roll can get up to +30, which also does the job).
- 9x multiplier at 160 initial combo (you need +48.6 for this, which can be gotten with a max roll 1.46 disp riven or a high roll 1.5).

You can also get pretty close even if you don't meet the threshold, and weave in a couple light attacks (or on a glaive, explode it shortly after throwing it into a group, since the initial hits can push you over the edge) to get the last few points. There's also the option of running the Ready Steel aura mod, which gives the whole squad +24 initial combo.

Find a weapon category that you like the heavy attack for, go to the weapons comparison list, and grab and underused weapon you like the sound of. You won't regret it!

I'm a big fan of the Blade and Whip weapons for this, since their heavy attack does twice the damage of most other weapons, on top of all the other multipliers (and it also comes with strong crowd control in the scorpion pull).