As powerful as Mimic Tear can be, it has to be able to use your build.
These are my personal favorite spirit ashes. If I’m going to use spirit ashes, I’m probably using one of these:

• Greatshield Soldier Ashes: The Guys! My nigh immortal troops. These guys just stay behind their shields, throw granades (that can proc frost bite) and bash with their shields. Bosses don’t know how to deal them. Nothing is funnier than watching Gaius’ little mind melt when trying to deal with them or watching Ancient Dragon Man get bullied in a corner. They are also decently cheap to summon at 74 FP.
• Jolán and Anna: A true endgame summon. It’s worth doing Metyr’s entire questline just for this summon. These two are mobile NPCs that do a lot of bleed build up. While they aren’t tanks, they are decently tanky (They survive Miquella’s orbital laser no problem). Some enemies you can simply gank with these two. Anna is also really good at drawing aggro with her ranged attacks. They are quite expensive to summon at 144 FP; drink your blue juice.
• Stormhawk Deenh: My personal favorite summon. This gives you immediately after summoning an insane aura buff: 20% more physical damage and 20% more stance damage which is, again, insane. Otherwise this summon is rather squishy and it’s pecks don’t do a lot of damage - it has a dive which it rarely uses that does a bit of damage and stance damage. Some bosses really struggle to hit him since he’s flying and is immune to ground moves. He’s super cheap at 47 FP. That’s cheaper than the starting wolves. At RL1 you can summon him and use Cragblade two times.

Why did I write an analysis of my favorite spirit ashes under a funny meme?