My recommendation is the Philips Norelco BT5275/41 5100 Beard Trimmer. This is based on a few really nitpicky requirements that I had:

1. Works in corded mode. Cordless is great, but eventually the battery dies on these things or I forget to charge it and need to use it in a hurry. I can't wait hours for the thing to charge (ruled out the Panasonic ER-GB40-S)
2. Single adjustable beard comb. The Wahl Peanut has been recommended here frequently, but I'm not a fan of keeping track of multiple attachments.
3. Ability to use the trimmer without the beard comb for defining lines. You'd think this is something that every trimmer would allow, but turns out the blade on the Philips Norelco QT4070 doesn't protrude and is sort of recessed so it's basically impossible to use it directly against your skin. No idea why they'd design it that way but was obviously a dealbreaker.
4. Self-sharpening/no-oil-required blades. I know oiling isn't that big a deal, but it's an extra maintenance step that I didn't want to have to deal with.
5. Powerful motor. I didn't want something that requires multiple passes over the same area. (ruled out the Braun Cruzer 6)

Honestly, even though I use and recommend the BT5275, I still long for my old trimmer, the Remington MB-50. It was discontinued a few years ago, but man that thing was awesome. Lasted me nearly a decade. They made a new model, the MB-200, but it is total shit. Doesn't work in corded mode and the non-replaceable battery dies after like 6 months. I miss my MB-50 :(