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I'm assuming the arcade versions, but what is the best port version of Gradius? And Gradius II?

I'm trying to narrow down my backlog.

Thank you so much for your attention

For Gradius 2, I'd say arcade is probably the best. With console ports the accuracy is here and there, but they can be a bit more easier if that is what you are looking for. NES version is more of a remix and MSX is a completely different game.

For Gradius 1, things get a bit more interesting. The arcade version is actually somewhat clunky, so the console ports can be pretty good choices too. I do like the NES version, but I don't know which version other than that could be a good choice. MSX has some extra stuff, and so do some other ports (Most notably that bone stage in some ports) and I feel like some of the more "accurate" home ports actually iron out the odd engine of the arcade port. You probably want to ask someone else about this.

Gradius 1 should be somewhat easy to complete however if you have played older horizontal games before, so even the arcade version should be quite a quick clear.

Saturn and PS1 got great ports of both.

this.

Deluxe Pack ports are not Arcade perfect, but they are good ports for their time. Most faithfull ports are probably Arcade archives.

Deluxe Pack is a great port of both games in terms of arcade accuracy - but if you want "the best version" of those two games, don't sleep on the PC Engine versions!
Both have an extra new stage added, and a bunch of rebalancing and adjustments. Konami really put a lot of effort into their PCE ports.

Aside from cosmetic differences (final stage in DP has grey palette as opposed to blue in AC), the turrets in final stage won't fire single bullet at all even in hardest dips and I recall Gau's laser attack are more randomized too.