The new Kehot siddur has a good starting point for those who need an English translation that focuses on being conversational, with enough of an explanation to provide some depth. On the online sample at the link, they have the first page of Shemonei Esrei (both the Hebrew side, and the english translation side) so you can see specifically if it is what you are looking for.
For those who already comfortable with Hebrew, for depth there is nothing I have seen like the Siddur Shai Lamora (although the online sample at that link is only about the first blessing of the morning). There are couple of caveats with it though, which may be good or bad, depending on what you are looking for. It is very wide-ranging, exploring kabbalistic and simple meanings. So the simple meaning is there, it just isn't the only thing on the page. Second is that it relies on what was said previously. So if you skip straight to Shemona Esrei, you won't get everything it said, for example, about the structure of a blessing in general and what it means, or what Hashem's name means, things like that.
A more intermediate alternative there is a siddur printed by Kehot (at least it had the Kehot logo on it) where it printed the Siddur with an interlinear hebrew elucidation. I can't find that one online (it was probably printed by their Israel division), but a local book store that carries Chabad items may have it.