1. Whoever is the friendliest (unless they are messing up their job catastrophically). Being friendly in a group should be the default behaviour for every single MMO player, but for some reason, it isn't, so I reward it. Conversely, even the most flawless, breathtakingly superiour player will not get a recommendation from me if they are an asshole.
2. Whoever held their own best in less than ideal situations. Like, say, a pull goes haywire and AOEs fly everywhere, but the healer keeps everyone going without a complaint. Or someone accidentally aggros an extra group and the tank just takes them on like a champ even though they already fought something. Or the tank dies against a boss, and the DPS takes over for victory etc. etc.
3. If nothing stands out at all, I prefer giving it to a solid DPS or a Tank. DPS rarely get recs, and tanking is such an extremely stressful thankless job.