I'll probably go for the Maus line as the free tier 10 line, but I feel like I won't enjoy the gameplay of the Maus and end up wishing I'd gone for something unusual like the Foch B.
For the last couple of weekends I've been hammering boosters and using some blueprints, plus the 40-odd days of premium account that I've been given for free have helped me get to tier 9 on the Strv. 103b line and tier 8 on the Type 5 Heavy line. I thought the O-I was just rubbish, but the O-Ni and O-Ho have actually been fun at times, especially when you meet clueless people. I'm at tier 7 on the Canopener line, but I'm kind of stalling on it because I don't really utilitise the double-barrelled aspect of it, so it is kind of pointless gimmick for me.
There are a number of tier 10s I've had researched for a long time, including M48 Patton, which I've always been put off buying because I always come across comments saying it is average at best in WoT of today. I actually don't have anywhere near enough credits to buy all of the tier 10s I have researched...