If you want to talk about a certain action as a concept, like in

             ______________(Swimming) en iyi antrenmandır

which verb form do you use. Is it just -mek-infinitive?

-me or -mek is possible but -mek sounds better:

Yüz-mek en iyi antrenmandır. (This makes me think that I have no comparison purpose to some other activity in my mind when I utter this sentence.)

Yüz-me en iyi antrenmandır. (This makes me think that yüzme is better than some other activities like "koşma" yürüme" "dans etme" etc.)

First one sounds more generic and the second is a bit specific and suggest that there is a context of the sentence.

It´s more simple than I thought. Thank you.