Statistically speaking, yes. Arceus currently has the highest base stats of any pokemon, at 720 total. However, this alone wouldn't make him the "best" pokemon. There are too many situations, and too much subjectivity to determine a "best" pokemon. Though he has the highest total base stats, there are still pokemon that outdo him with individual stats, such as blissey and HP, Rampardos and attack, Alakazam and special attack, Steelix in defense, Deoxys in special defense, Ninjask in speed, and many others. Though he can do a lot of jobs, he won't be doing everything; he'll usually be put in as a wall, sweeper, tank, etc, meaning he's still in competition with other respectable pokemon. He also has a wonderful movepool, but remember that in the end, every pokemon is limited to four moves. You also have to account for abilities, the current competitive environment, which type you pick for arceus, and several other things.
I'm not trying to Downplay Arceus or anything, it's obvious he's one of the strongest pokemon out there. If going by just base stats, then yes, he's the strongest. But "best" pokemon holds a different definition for everyone; he can fit almost any role you need him to. Also note that he wasn't around in Gen I-III, so there's no way to say anything about how he did then.
