Ukraine

Not particularly big, no. But neither is the Russian one, and it's fairly modern.

The main issue is they can offer to escort civilian trading vessels. If Russian forces attempt to interdict and, even by accident, attack one of the Turkish warships, that is enough justification to trigger Article 5 if Turkey wished, and pull NATO wholesale into the conflict.

While we probably wouldn't actually attack into Russia for fear of getting nuked, we could turn economic sanctions into an international blockade and eliminate every single Russian asset not hiding in Russia with warlike prejudice. Then just wait for the Ukrainians to beat them.

Turkey has the largest army in Europe at the moment with over 250k soldiers and is the second largest navy in the ~~baltic~~ black sea, maybe the strongest one actually.

(Edited, wrong sea mentioned)