My opinion:
30 Days - High Khatun - This is a great champion for just about anyone. She may not be the very best but she's darn good and you get her guaranteed for free. She's a great lead for Arena, Dungeons, and Campaign when you're still getting your footing in the game. She offers that wonderful speed buff with her A2, a slow debuff on her A1, and some turn meter manipulation with her A3
60 Days - Yaga the Insatiable - This champion is so so, but if you're a free or low paying player, this champion isn't bad as your first Clan Boss Poisoner if you don't have someone better like Frozen Banshee.
90 Days - Dark Athel - While I personally feel like the starter champion Athel is better, Dark Athel has some worthwhile benefits like her A1 and A2 debuffs.Still she's not exactly amazing as the final champion of the initial tier of login rewards.
120 Days - Dark Elhain - This champion isn't bad on her own but shines against Tormin in the Arena and the Ice Golem dungeon. She might be a bit niche but overall I think she's a pretty good champion.
150 Days - Lordly Legionary - This guy would have been great for Fire Knight... if he wasn't Magic Affinity. He's not a bad champion but he's a bit too gimmicky for my taste due to his conditionals.
180 Days - Scyl of the Drakes - This champion is transformative! Her kit is amazing IMO and even if she's at an affinity disadvantage it hardly matters. Constant party healing and single champ speed buffing, even if she's CC'd. A double hitting aoe that has a good chance to stun. A resurrection ability with ally protection built in. An A1 that not only places a speed debuff, but also reduces the turn meter! She's a true winner for the final champion of this tier of login rewards.
I will say that out of the 6 champions we have so far from Login rewards, 3 are mediocre, 1 is pretty darn good if a bit niche, and 2 are downright amazing. The new ones coming in the future I can't really get too into since I haven't seen their base stats yet, but the 270 Day Void Legendary doesn't seem terrible. Turn meter manip A1 seems nice, especially on a void champion so affinity isn't an issue. Just the fact that she's Void Affinity and Defense based makes her decent in my book.