I have mainly been using Claude 3.5/3.6 Sonnet via API in the last several months (or since 3.5 Sonnet came out). However, I was using o1 for a challenging task at one point, but last I tested it had issues with some extra backslashes for that application.

I also have tested with DeepSeek R1 and will test some more with that although in a way Claude 3.6 with CoT is pretty good. Last time I tried to test R1 their API was out.