First of all I know I'm not actually answering your question but you're going to find various opinions on this concept. For instance love spells of this nature involve meddling with free will. Many find this immoral and shy away from such spells. I on the other hand simply don't believe you can interfere with free will so I do not believe love spells truly work unless both parties want to be together already. Of course then you wouldn't need a love spell. There are plenty of who do cast love spells but a spells effectiveness is determined by the caster not the spell itself so I'm not certain anyone can give you a specific "very effective spell" though they may have a favorite to recommend.
something I recommend for people desperate for a love spells is something a little subtler: a honey jar spell. These can be used in many circumstances, not just love, and are used to 'sweeten' a person up to you. So essentially, to get your ex back to you, you still have to put the work in (meaning you aren't meddling with free will and such) but they just get on better with you. This can make smoothing over rough areas a lot easier.
there are many honey jar spells on here(and you don't need to have physical parts of the desired person, often a photo and a name is enough).
I personally don't believe there are effective or strong spells. Spells are just words. The power is 100% you. So this means you need to research, learn, and practice. This may mean doing spells multiple times and tweaking them here and there. Just don't give up. Spells are primarily intention nothing more so as long as you are strongly projecting your intention and visualizing what you want it will manifest. But you have to believe in yourself. Also learn to let go. When you do a spell just forget about it because you know it will work. It's just about waiting for it.
To actually answer your question I think the strongest spells are the ones that are the most simple. Write a petition, light a candle; or burn the petition; or bury it. Simple.
No I write my own spells on the fly. Like I mentioned before it's not the spell that makes it work its you visualizing and willing it to be. Any spell you find or make will work the same assuming you put the same amount of effort into it.
Name of spell:'Moon of grandure, tomb of fallen chances the sauntered one who entered my life."
Spirit: lilin,undine, or sylph
Tools :piece of wood,silver,and bowl half filled either water
Ritual: place the wood in one hand and silver in other then
Place them down on either side of the bowl.
Incantation:neh veh nah veh ta nah