I thought about this recently.
I have thought about this recently.
I was thinking about this recently.
What tense should i use?
I thought about this recently.
I have thought about this recently.
I was thinking about this recently.
What tense should i use?
What tense should you use? That depends on what you want to say. All of them are correct in some context.
Simple past, "I thought about this recently.": This means that some time in the recent past, you thought about this, but the action is completed and finished. This one is a pretty good choice. You might use it if yesterday you were thinking about it, and then you finished your thinking, and you haven't thought about it since.
Present perfect, "I have thought about this recently.": The present perfect is used for unspecified times in the past, and relating your past actions to the present time. If you say "I have thought about this recently", it implies that maybe your thinking is not yet complete, or maybe you thought about it on several occasions.
Past continuous, "I was thinking about this recently.": The past progressive is used to indicate an action that was ongoing at some point in the past. This is probably the worst choice for a stand-alone sentence, because we usually use it to refer to a more specific point in time; if all you want to say is that you were thinking about this, then the simple past or present perfect is probably a better choice. If someone just said to me "I was thinking about this recently", I would want to say "...and? Then what?" because we usually use it in a context like "I was thinking about this recently when I realized something important."
Here are a couple references for tenses: