You have to remember that Garth Ennis was in his early to mid 20s when he wrote for 2000 AD, so he can be forgiven for some substandard strips now and then. But I think the majority of his work was pretty good, and there were some outstanding g stories like Raider and Death Aid. Most importantly, he understood Dredd's character and voice, which all the other writers in the '90s didn't (except Wagner obvs).
I agree with most of Magnetica's list of shame, and I'd also add Trash and Dead Meat. But I would defend Indigo Prime and Leatherjack, which are brilliant, but I understand that Smith is not everyone's cup of tea. As has been said already, the first series of Time Flies was alright, and I'd rate it higher than Hewligan's Haircut.
As for Slaine and Finn, while I can see the similarities I think they are also different enough that there was room for both in the comic. That may have just been an excuse the editor used to get rid of a strip he didn't like. Personally I enjoyed Finn more than Slaine. The first two books of Finn are outstanding (and I really liked that artist too; whatever happened to him?) so it was nice too see them reprinted for the first time in the floppy recently. The later ones weren't as good but they were alright.
Kola Kommandos wasn't so bad, it was just aimed at younger readers. I don't want to see it come back, but not every strip should be intelligent and dark, you've got to have something to keep young readers wanting to read it.
The Zaucer of Zilk was fantastic.