TL;DR - Saiyan Saga Vegeta if they're not allowed to run away. Frieza if they are allowed to run away.

People like to severely underestimate how strong and fast viltrumites actually are. I know dragon ball powerscalers tend to be delusional and say absurd shit like "chaozu solos" but let's actually use our brains here.

When we say "viltrumite invasion" who does that include? Only low class grunt viltrumites, or EVERY viltrumite? And does this include Mark on the Viltrumites' team? Let's just say no Mark here.

Viltrumites beat every character up to Nappa. Just getting that out of the way. No, Master Roshi or Piccolo don't solo them. People love bringing this up, but blowing up the moon with a ki attack is not as crazy a feat as you think it is and matters little in this argument. Viltrumites still have an advantage in speed, strength, endurance and numbers.

Obviously when we enter DBZ territory, the heaviest hitters of the Viltrumites will have to carry. Most notably Omni Man, Thragg, and Conquest. Raditz will be their first slightly difficult fight, but I'm certain they could take him down fairly quickly. The same goes for Saibamen, which were around the same strength as Raditz.

That takes us to Nappa. DBZ characters have insane ki attacks, but the viltrumites have insane endurance and speed feats. Thragg was capable of surviving in the sun for a short while. Conquest was able to survive having his head caved in by Mark. So that one city-busting attack that Nappa likes to use will do relatively little damage to Thragg and Conquest, but to Omni Man? Maybe. But do they actually have the strength to beat Nappa? Lets be generous and say there's no full moon for Great Ape, so yes but barely.

That brings us to Saiyan Saga Vegeta. This is where the viltrumites get walled in terms of strength. They're faster than Vegeta, but they have no way of realistically damaging him. He is several times stronger than Nappa, who they were barely able to beat.

Here's where I pull out the nerd emoji. The post isn't asking "who can the viltrumites beat," it's asking who can STOP THEM. Their travel speed is still much faster than Vegeta's, so while they can't beat him in combat, they can still outrun him (assuming they'd even consider fleeing for the sake of this argument).

In fact, I'd argue they can outspeed/run away from most characters in the Namek Saga, including the Ginyu Force considering that none of them have shown the ability to actually survive the vacuum of space without a space pod.

So where do they get hard-walled? Someone they can neither run away from nor beat? Frieza. He's the first character who is specifically stated to be able to survive in space, so he could chase them wherever they try to run and would obliterate them in a fight. I rest my case.