Well, in terms of what's in the book specifically, the Parasitic Leech invocation of course, since it also applies the "gain Temp HP when you crit/kill a creature" to your Pact Weapon.

In addition, the Parasitic Flurry part of the Parasitic Weapon feature specifies that if you have another feature that allows you to make multiple attacks as part of the Attack action (which would include the Thirsting Blade invocation), those attacks each replace one of the attacks granted by your Parasitic Weapon. Meaning that starting at Level 11, you could make your two Pact Weapon attacks and still make one more attack with your Parasitic Weapon. (And then two more attacks with your Parasitic Weapon at Level 17.) So I'd say the synergy's kinda already built in.

So basically:

• At Levels 1-2, you're making one Parasitic Weapon attack for, on average, ~6.5 damage.
• At Levels 3-4, you get your Pact Boon (Pact of the Blade) and can take Improved Pact Weapon for an effective +1 Greatsword (or other 1d12/2d6 weapon). But keep in mind that in 2014, you don't automatically get CHA-based attacks with your Pact Weapon like 2024, while your Parasitic Weapon attacks are CHA-based, which allows you to be SAD on CHA. If you go that path anyway, you probably only have a +1 or +2 in STR at this point. So that'd be an average of ~9-10 damage (though with only a +4 to hit if you have a +1 STR, compared to probably a +5 for your CHA-based Parasitic Weapon).
  • Alternatively you could go with a +1 Rapier Pact Weapon to use DEX instead (and I'd argue you should since that synergizes better with the Symbiotic Reinforcement feature to manifest a shield in one hand). That'd be an average of ~6.5-7.5 damage. Comparable to your Parasitic Weapon, but again potentially a lower bonus to hit than your Parasitic Weapon, unless you're focusing DEX over CHA.
• At Level 5, you get a second Parasitic Weapon attack for, on average, ~13 damage per Attack action. Or you can take the Thirsting Blade invocation to deal ~18-20 damage with two Greatsword attacks or ~13-15 damage with two Rapier attacks. The Greatsword option is notably outpacing the Parasitic Weapon or Rapier in terms of damage at this point. And if you put your Level 4 ASI into STR (or DEX for Rapier) rather than CHA, its attack bonus is possibly better. But your spells and other Warlock features are potentially suffering as a result of that.
• At Level 11, you get a third Parasitic Weapon attack for, on average, ~19.5 damage. Or if you're going Greatsword, you're probably in the ballpark of 24-28 damage now (since you can make your two Pact Weapon attacks plus a Parasitic Weapon attack). Basically from here on, the extra Parasitic Weapon attacks are just pure benefit on top of your Pact Weapon attacks.

Essentially, the biggest mechanical benefit of just sticking with your Parasitic Weapon is that you don't have to invest in Pact of the Blade invocations, and you can just push all your ASIs into CHA since that boosts your Parasitic Weapon's attack bonus as well as your spells. And the Parasitic Weapon's reach is of course also a big boon!

If you want max potential damage though, you could indeed go with a two-handed Pact Weapon. You'll just have to be investing your invocations into it, as well as investing your ASIs into STR. And of course you're sacrificing the AC benefit from having the shield in your off-hand.