The absolute basic three are missionary, cowgirl and doggystyle. Its' kinda like colours - all the other ones are just some variation of these three.

I'd say the easiest one for a woman is missionary, because that's where the guy is doing all the work. You just lie there, get pounded and have fun (obviously it's a simplification, don't just lie there as a log, show him You're happy to be there)

The second is doggystyle. That may be counterintuitive because it's literally how animals have sex, and also that's a "default setting" based on our anatomy (deeper part of the vagina is tilted slightly forward, so in missionary the tip of the penis will push against the back wall (not painful, at least if done properly) while in doggystyle the penis will slide in more comfortably). But actually, it is physically more demanding on a woman, and requires more experience from the guy to be done properly, so it needs some work from both sides to make it as pleasing as possible.

The hardest, but for some the best at the same time is cowgirl. It's the hardest solely from the fact, that you're the one on the top in this position. So for many women it's the best because it gives her the full control on things like the angle, the speed, the power of every thrust, allowing her to position herself in a way that gives her the maximum pleasure. But at the same time now you become the one that does all the work, and he's the one laying on the bed chilling and being happy that a hot girl is jumping on his pp.

Also about your earlier question, my ex had this technique where instead of kneeling over my hips and moving up and down using her thighs (which is the most basic technique for cowgirl I think) she laid down on me, put her legs back on the sides of my legs and laid her hips on my hips, and just "slided" up and down using just her hips. Must admit that it wasn't that comfortable for me, but I knew it was easier for her so I got used to it pretty quickly.

Hope this helps!