My favourite is Quake 3 engine. Very common, stable, fast.
And how many games were started thanks to existence of this engine. (actually how many games were started due to Id software
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)
I like Source too because it uses such cool effects as you guys already said, also textures are looking great in source engine. (or they all have some shader filters ?)
The engine I really don't like is the Oblivion. Tes IV: Oblivion engine is great for terrain, plants, vegetation stuff - and quite good physics (but there are few bugs unfortunatelly) but I think that it is optimised very badly. All of my friends who played Oblivion had many technical problems with the engine including graphical bugs/crashes to desktop and such. Also the biggest disadvantage with the engine I think it's some bad memory allocation. After one hour of playing game tends to do some strange fps jumps and sometimes swaps hardcorely.
I understand that it's engine of the next generation but maybe it would be better if they optimise it a bit more than that. (and this nice HDR eats system a lot that's why my friends are playing with Bloom only)
PS I'm from Poland so maybe I have differ feelings than people from US and other countries, because a common system in Poland is : 2 ghz, 512 mb ram, 128 mb video card , while in US most people use twice better specs like those I guess.