What's the best Knight move for White against this variation (loved by beginners) in the Philidor Defense?
[FEN ""] 1. e4 e5 2. Nf3 d6 3. d4 exd4 4. Nxd4 c5
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As Ian Bush already mentioned in a comment, 5.Bb5+ is arguably the strongest reply. After 5....Nd7 6.Nf5 or 5....Bd7 6.Bxd7+ Qxd7 7.Nf5, black has some serious problems with the weak pawn on d6 and square d5. Note that there are some similarities with a line in the Sicilian: 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 e5 5.Bb5+.
Other moves are less convincing. After 5.Nf5 Nf6 or 5.Nb5 a6 6.N5c3 Nf6, black is probably fine. 5.Nf3 is not a bad move, but it doesn't take advantage of black's weaknesses.
[FEN ""] [StartPly "1"] 1.e4 (1...c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 e5 5.Bb5+) e5 2.Nf3 d6 3.d4 exd4 4.Nxd4 c5 5.Bb5+ (5.Nf5 Nf6) (5.Nb5 a6 6.N5c3 Nf6) (5.Nf3) Bd7 (5...Nd7 6.Nf5) 6.Bxd7+ Qxd7 7.Nf5