Naas is probably the best all round town in the county.
However if access to Dublin is a priority it would be Leixlip, Maynooth, Celbridge in that order as all are on the Dublin Bus and Dublin Commuter rail service. The Dublin Commuter rail prices are far cheaper than the services is Newbridge etc.
Celbridge only really has the 67 bus. Hazelhatch station is a bit too far outside the town for regular use unless you drive to it, and to make it worse the train only goes to Heuston.
Maynooth has the 66 (and the 67, but it takes too long to get to the city centre to be usable), and a proper train service in to Connolly.
Leixlip has by far the most comprehensive bus service with the 66, 66A and 66B. It is also the only town in the county with a decent Nitelink service: every half hour on Fri/Sat nights, and the whole town is covered. Leixlip also has two train stations, again directly into Connolly.
A comparison of return rail fares:
Maynooth to Connolly: €6.00 (Leap or IE smartcard)
Sallins to Heuston: €14.40
The train from Hazelhatch - Heuston takes 20 mins and costs €2.40, the Luas or bus will have you in the city centre then in 10-15 mins. That Maynooth train is a bit of a Mumbai express and takes a good 50 minutes. €6 with little chance of getting a seat doesn't sound great to me!