Hrmm, worst Space Marine Chapter. Let's see.
Salamanders-No, they couldn't just have someone with a dark skin color! They had to have their skin LITERALLY BURNT BLACK by the sun! Yeah, that's pretty stupid. However, the rest of their fluff is good and stuff to distinguish them from other Chapters, so they aren't the worst.
Blood Angels-Alright, every Loyalist Primarch has died, yet this is the only Chapter who went emo as a result. The explanation behind it is generally vague and poorly thought out, but to be honest that can be said about a lot of things in
40k. This is their defining trait, so that makes them a candidate for worst Chapter, but it isn't too bad so they're a Dark House.
Ultramarines-These guys were built off of a fine concept; mostly generic Marines with a Roman Theme. But they made them into ultra Mary Sues. I can't think of a single time they suffered a major defeat and didn't still win whatever war/overall battle they were in. Hell, I can only think of a single defeat they had. Due to this, they're a strong candidate for worst Chapter.
Space Sharks and Rainbow Warriors-Alright, these names are terrible. However, they are so over the top and were down that way on purpose so that I don't think the names are actually that bad. As a result they aren't a contender, but they easily could've been.
Space Wolves-I've explained earlier that they're hipsters. I don't feel like explaining it again, but I'm sure I'll do it if someone asks. In addition, they use the word "Wolf" way too much. This puts them as a strong contender alongside the Ultramarines.
Hawk Lords-Even Slaanesh worshippers aren't allowed to have a colorscheme this bad. However, they have no bad fluff, so they're only a moderate contender.
Grey Knights-Similar to the Ultramarines, they were built on a fine concept and were indeed done well for awhile, but with their recent Codex they've strayed into Mary Sue territories. These guys are a strong contender aswell.
Doom Eagles-They're like the Rainbow Warriors, except their name wasn't a satire on terrible names. It actually was a terrible name. But that's their only flaw, making these guys a weak contender.
Mantis Warriors, Lamenters and Executioners-Heretics are never let off. Yeah, I know they were treated badly, but normally the Imperium would've obilterated them and then obilterated the dust. Thus these three Chapters are a moderate contender.
Dark Angels-Used to be good, but their secret is stupid and uncreative at all. Woop
de doo, some of your Chapter fell to Chaos, SO DID EVERYONE ELSE ACCEPT THE GREY KNIGHTS. You aren't unique. Having their defining thing be something stupid makes the Dark Angels a strong contender. And the winner is....