In my opinion, the best results come from using WeldOn. Big caveat though…WeldOn is extremely tricky to use and takes lots of practice. It’s water thin and must be ‘wicked’ between the pieces being attached…and even then only at the seam. If it goes too far out, it can be seen as well. If done right and carefully, it literally can make two pieces of acrylic into one piece.
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If I could only choose one, it would be 3M 468MP 
I use a hard plastic spatula that came with a roll of vinyl or spackle or something. I apply it before cutting, and I make sure to squeegee the bubbles as best I can. Some I pop with a pin and go over again with the squeegee. I’ve been doing it so long now that I’m quick.
If you are stacking transparent acrylic, you should use something like weld-on. If you are wedging it between layers of other stuff or are using non-clear acrylic, you won’t see it, and it looks great.
I use it in projects that require glue. Here I have it keeping the 2-tone acrylic to the wood, and holding the base layers together.
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