Obviously Sceptile, since it's my favorite Pokemon. But since I've already spoken about it on day 1, I'll take this opportunity to talk about the other one that switches out with Sceptile as my favorite Pokemon sometimes, Cyndaquil.
First off, how can you not love this thing? Easily the most adorable Pokemon. For me personally though, Cyndaquil was my very first Pokemon, back when I was like 5 or 6, 20ish years ago. I'll admit nostalgia plays a factor here but even today whenever I revisit Johto I always make damn sure to save a spot in my team for my Cyndaquil and its evolutions because I love them all so much.