^This. In a decently coordinated group, Herald is one of the easiest titles you can get imo. The fight lasts maybe 5 minutes and only has one or two Big Bangs before p2.
Hardest in terms of rarity due to an excessive grind would probably the 250K HKs title, Bloodthirsty or w/e.
Hardest in terms of pure skill would definitely be the Season X Glad title. Raid titles (Firelord, SoA) can be outgeared. Hardcore arenas are a different ball game entirely, especially at that level.