It's difficult for me to give an honest rating since I have played Touhou for long enough to be able to handle most extras fine, but I would say Suwako and Okina are probably the easiest extra bosses for a newer player, while Koishi, Evil Eye Sigma, Momoyo, and Hecatia are probably the hardest (Hecatia in particular is my vote for hardest extra boss in general, provided you've gotten used to SoEW's mechanics for Evil Eye Sigma).
However, Suwako is only the easiest once you learn her - she is the most trick-based extra boss in the series, and going in blind is likely to lead to a lot of game overs until you learn what those tricks are. Koishi is also like this on the opposite spectrum, being one of the hardest extra bosses to beat for the first time, but arguably one of the easiest to perfect in the long run. My recommendation, honestly, is to try them all and find one you gravitate towards, then focus on her until you can do it. That's how I got my first extra clear with Ran way back when. When you're starting out motivation is more important than subjective difficulty.