Villian in this hand is "sputnik3000" I have about 1,300 hands on him and he runs at 15/12/2.89. I'd say he's a winning player. He has 3bet me a few times already (on various tables) and i've been pretty active at this table (23/18 over 200 hands, more aggro in the past 30 hands or so because i've been getting good cards).

Anyway on to the hand. I think the flop call is standard (given my PF call) - I wasn't sure whether or not to bet the turn however. I decided on checking.

He bets the river and I had a hard time putting him on a hand. Call or fold?

I would fold here as played. I probably would fold to his flop bet too, 18/12 is kind of tight/nitty. I think his 3 bet range OOP is like TT+, AK, so no set no call on flop.

Looks like AK to me...maybe QQ...not KK/AA and only possibly AJ...I can find a call.

Even I find a fold here most of the time.

I think a turn bet here is hot, because you can expect QQ-AA with no diamond to make a Phil Hellmuth laydown a respectable percentage of the time.

I called the flop because I thought there was a decent chance 99 is the best hand considering he had a huge 3bet range. On the turn, I figured by betting I would get all worse hands to fold and better hands to call/CRAI. I didn't really think i'd get to fold out a better hand. I also think he'd bet the turn with an overpair. On the river, my thought process eliminated big PPs from his range and I didn't think he'd valuebet AT or AJ here. I couldn't really put him on a hand, so I called. I was right on the fence though.

I totally agree with your thinking, the only thing I disagree with is him not folding a better hand. I think if you make a 2/3 pot bet he is probably going to have to at least consider folding a hand like 2 black Q's, K's, A's especially knowing his is probably going to have to call a bigger bet on the end. Also by betting the turn you allow yourself to possibily push him off A or K on the river if the flush draw doesnt hit. Sure you push out the hands that your are beating but most of those hands are AK AQ, KQ where he has a lot of outs especially if he has a diamond he is looking at 15+ outs. I am totally happy winning the pot right there if I can. I like the river call a lot, he either hand a big hand or a total bluff, figure you win 50% of the time.