Season 2 was definitely the strongest season. I'd say Season 6 was the weakest.
I thought there were a few too many stand alone episodes in Season 2, but regardless... I too think it's the best season. Here is how I'd rank the seasons of Buffy...
Season 1 - 6/10
Season 2 - 8/10
Season 3 - 7/10 (I hated Faith, otherwise this would have been higher)
Season 4 - 7/10
Season 5 - 6/10
Season 6 - 2/10
Season 7 - 5/10
Season 6 was absolutely horrible. Season 7 would have been much better without the potentials and the group turning on Buffy; I also didn't like the finale.
But, overall, it's definitely worth checking out. The show at times could be very endearing and exciting. Plus, there are some episodes that are just some of the greatest television ever done (Surprise, Innocence, Becoming, Doppelgangland, Hush, The Body, and Once More with Feeling).
Angel though, that series is fantastic all the way through, with the exception of the Jasmine arc at the end of Season 4, and some of the Kate Lockley episodes in season 1.
So when Angel starts his gets his own show, how should one it and Buffy (as I know there are cross overs in various episodes of both)
Start with Season 4 of Buffy, watch the first episode and then watch the first episode of Angel. And then just rotate.