Mummelmann on 29 May 2010
Father Grigori is at the end of the Ravenholm level, I'd say somewhere around the middle but I can't seem to remember since its been a couple of years since I played it last.
The driving levels (both boat/jet ski and car) are what take the longest but there's a lot less content than in the levels you traverse on foot so its a bit deceptive.
By far the best level in HL2 imo is Nova Prospekt, which is after the Ravenholm level, you should really play it! 
Episode 1 is slow paced and focuses more on bonding with Alyx while Episode 2 is epic in every sense of the word, its manic, chaotic and exciting as all hell!
