I would have to say F-Wara... his instant overheads are just too spammable.
On the other hand, it depends on what you mean by
best characters for mixups?Most spammable? F-Wara instant overheads, H-Len fuzzies, F-Kouma poke-throws
Hard to react to? All fuzzies I guess, and tight sandoori's (e.g. Satsuki)
Mixups you are forced to block? C-Koha (stay under plants), F-Aoko (Blue Fiyah), F-Maids etc.
You might also want to take a look at
this thread.