TLDR: All FOSS BitTorrent clients are shit. Stupid UI changes such as icons instead of adding features. Use Tixati.

Halite
Not updated, *alpha*-0.x-quality.

Tixati
Good, portable.

Tribbler
No portable version.

Transmission
No portable.
"Early preview" bullshit.

Deluge
Not portable.

Writes settings to: Application folder
Stealth: ?  No. Temporary files are saved to %APPDATA%. For more details, read here and here.
License:  GPL
How to extract: 
Download the installer and extract to a folder of your choice
If present, move the files and folders inside $_OUTDIR to the main folder (where deluge.exe is located)
Launch deluge.exe -c data

LFTP
CLI only.

qBitTorrent
Too simple.

Shareaza
Not portable.

FrostWire
Ads (read: malware), Java-based. Not portable.

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